Remove unused SecretRedactor registry and update stale comments (#574)

The exact-value secret registry (`SecretRedactor`) was built as
infrastructure ahead of its wiring, but the wiring was never merged —
the team settled on content-based redaction (entropy analysis + gitleaks
patterns) as the sole mechanism. The type had zero consumers outside its
own crate. This PR removes it and corrects two doc comments that
described the abandoned design as pending.

**What changed:**

1. `fabro-redact/src/secret_registry.rs` deleted in full (~217 lines),
with its `mod` declaration and `pub use` re-export removed from
`lib.rs`. `Region`, `redact_string`, `redact_json_value`,
`DisplaySafeUrl`, and everything else in the crate are untouched.
2. The `resolve_extra_headers` doc in `fabro-auth` no longer promises
future exact-match registration. It now honestly states that low-entropy
header values not shaped like credentials are not caught by
content-based redaction.
3. The `InterpString` module doc in `fabro-types` no longer describes a
pending per-run registry. It states the real architecture: resolved
secret values are plain strings, and redaction is content-based applied
at output serialization.

**Known limitation (pre-existing, not introduced here):** a declared
secret whose value is a low-entropy ordinary word (e.g. an environment
name) is not caught by content-based detection. This was the gap
`SecretRedactor` was meant to fill; it is an accepted trade-off, not a
regression from this PR.


### Fabro Details

<details>
<summary>Ran 8 stages in 26m 21s for $2.27</summary>

| Stage | Duration | Cost | Retries |
|---|---|---|---|
| start | 0s | – | 0 |
| toolchain | 1s | – | 0 |
| preflight_compile | 2m 34s | – | 0 |
| preflight_lint | 2m 45s | – | 0 |
| implement | 0s | – | 0 |
| simplify_fable | 9m 44s | $2.27 | 0 |
| simplify_gpt | 0s | – | 0 |
| verify | 10m 50s | – | 0 |
| **Total** | **26m 21s** | **$2.27** | **0** |

</details>

<details>
<summary>Ran <code>ImplementPlan.fabro</code> (11 nodes and 14
edges)</summary>

```dot
digraph ImplementPlan {
    graph [
        goal="Implement and simplify",
        model_stylesheet="
            * { model: claude-opus-4-8; }
        "
    ]
    rankdir=LR

    start [shape=Mdiamond, label="Start"]
    exit  [shape=Msquare, label="Exit"]

    toolchain         [label="Toolchain", shape=parallelogram, script="command -v cargo >/dev/null || { curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y && sudo ln -sf $HOME/.cargo/bin/* /usr/local/bin/; }; cargo --version 2>&1", max_retries=0]
    preflight_compile [label="Preflight Compile", shape=parallelogram, script="cargo check -q --workspace 2>&1", max_retries=0]
    preflight_lint    [label="Preflight Lint", shape=parallelogram, script="cargo +nightly-2026-04-14 clippy -q --workspace --all-targets -- -D warnings 2>&1", max_retries=0]
    fix_lints         [label="Fix Lints", prompt="The preflight lint step failed. Read the build output from context and fix all clippy lint warnings.", max_visits=3]
    implement         [label="Implement", prompt="Read the plan file referenced in the goal and implement every step. Make all the code changes described in the plan. Use red/green TDD. Be sure to use the rust-style-guide skill to help you follow this repo's Rust style conventions.", model="gpt-55", reasoning_effort="xhigh"]
    simplify_fable    [label="Simplify (Fable)", prompt="@prompts/simplify.md", model="claude-fable-5", reasoning_effort="xhigh"]
    simplify_gpt      [label="Simplify (GPT-55)", prompt="@prompts/simplify.md", model="gpt-55"]
    verify            [label="Verify", shape=parallelogram, timeout="1800s", script="git fetch origin main 2>&1 && git merge --no-edit --no-stat origin/main 2>&1 && cargo +nightly-2026-04-14 fmt --all 2>&1 && cargo dev docs refresh 2>&1 && cargo +nightly-2026-04-14 fmt --check --all 2>&1 && { command -v rg >/dev/null 2>&1 || { echo 'rg is required for verify'; exit 127; }; } && ! rg -n 'AuthMode::Disabled|RunAuthMethod|RunSubjectProvenance|\bActorRef\b|\bActorKind\b|AuthenticatedSubject|AuthenticatedService|AuthorizeRunScoped|AuthorizeRunBlob|AuthorizeStageArtifact|AuthorizeCommandLog|auth_method\s*==\s*\"disabled\"' lib/crates apps lib/packages docs/public/api-reference/fabro-api.yaml 2>&1 && cargo +nightly-2026-04-14 clippy --workspace --all-targets -- -D warnings 2>&1 && cargo nextest run --workspace --status-level slow --profile ci 2>&1 && cargo dev docs check 2>&1 && bun install --frozen-lockfile 2>&1 && (cd apps/fabro-web && bun run typecheck) 2>&1 && (cd apps/fabro-web && bun run test) 2>&1 && (cd lib/packages/fabro-api-client && bun run typecheck) 2>&1 && cargo dev build -- -p fabro-cli --release 2>&1", goal_gate=true, retry_target="fixup"]
    fixup             [label="Fixup", prompt="The verify step failed. Read the build output from context and fix all format, clippy, Rust test, docs, TypeScript typecheck/test, and build failures.", max_visits=3]

    start -> toolchain
    toolchain -> preflight_compile [condition="outcome=succeeded"]
    toolchain -> exit
    preflight_compile -> preflight_lint [condition="outcome=succeeded"]
    preflight_compile -> exit
    preflight_lint -> implement [condition="outcome=succeeded"]
    preflight_lint -> fix_lints
    fix_lints -> preflight_lint
    implement -> simplify_fable -> simplify_gpt -> verify
    verify -> exit  [condition="outcome=succeeded"]
    verify -> fixup
    fixup -> verify
}

```

</details>

⚒️ Generated with [Fabro](https://fabro.sh)

---------

Co-authored-by: Fabro <noreply@fabro.sh>

Author: fabro-sh-fabro[bot]

lib/crates/fabro-auth/src/resolve.rs

@@ -477,9 +477,10 @@ impl CredentialResolver {
 
 /// Resolve a provider's `extra_headers` interpolation sources with `ctx`.
 ///
-/// Provider header secrets resolve outside the run-boundary redactor
-/// registration path. Keep this path free of value logging until exact-match
-/// registration is threaded through.
+/// Resolved header values may contain secrets; keep this path free of value
+/// logging. Content-based redaction covers credential-shaped values on output
+/// surfaces, but no mechanism redacts these exact values, so a low-entropy
+/// header value that does not look like a credential is not caught.
 pub(crate) fn resolve_extra_headers(
     provider: &ProviderId,
     headers: &HashMap<String, String>,

lib/crates/fabro-redact/src/lib.rs

@@ -8,11 +8,9 @@ mod entropy;
 mod gitleaks;
 mod jsonl;
 mod safe_url;
-mod secret_registry;
 
 pub use jsonl::{redact_json_value, redact_jsonl_line};
 pub use safe_url::{DisplaySafeUrl, DisplaySafeUrlError};
-pub use secret_registry::SecretRedactor;
 
 pub(crate) const REDACTION_MARKER: &str = "REDACTED";
 

lib/crates/fabro-redact/src/secret_registry.rs

@@ -14,9 +14,10 @@
 //! Resolution timing is split: `vars` substitutes early (server-side, at run
 //! creation) via [`InterpString::substitute_with`], while `env`/`secrets`
 //! resolve late, at consumption time in the process that owns
-//! the value, via [`InterpString::resolve_with`]. Declared-secret values are
-//! intended to be registered into a per-run exact-value redactor where secrets
-//! resolve; sensitivity is not tracked on resolved strings.
+//! the value, via [`InterpString::resolve_with`]. Resolved secret values are
+//! plain strings; sensitivity is not tracked. Redaction of run output is
+//! content-based (entropy analysis plus credential patterns), applied where
+//! output is serialized.
 
 use std::borrow::Cow;
 use std::fmt;