fix(catalog): admit a prepared plane without runtime workspace dirs

[schickling-assistant]

Problem

.workspace is runtime-only: the agent owns its contents, no declaration
describes them, and #425 made the raw preimage capture none of them. The apply
side never learned that. project_excluding still demanded that every declared
workspace fact already exist in the prepared plane, which makes the exact repair
path the raw preimage exists for unreachable — a prepared plane built from a raw
snapshot has no such directory:

$ st2 catalog digest --catalog <live> --prepared <raw-snapshot> --json
Error: inspect prepared workspace fact <prepared>/agents/host/worker/.workspace

Caused by:
    No such file or directory (os error 2)

This is not theoretical. It is currently blocking a dev3 supervisor repair: the
deployer raw-snapshots an invalid live catalog, publishes valid declaration bytes
into that snapshot, and applies against the opaque preimage. On main (fbd1ff6)
that sequence dies at the digest step for any agent whose declared workspace
directory does not happen to exist.

Goal

A prepared plane may omit the directory for a declared workspace fact. Every
other rule holds, and the applied catalog still satisfies the live invariant that
a declared workspace fact is present.

Decisions

• The fact stays in the projection. Only the pre-existence requirement is
  dropped. materialize_projection already publishes workspace_dirs, so a
  missing one is republished as an empty directory and rootSha256 is unchanged
  — the fact is hashed from the declarations either way.
• A present directory must still be real and empty. Prepared state may not
  smuggle runtime bytes (or a symlink) through the declaration plane. That
  rejection is untouched and still covered.
• Uniform across prepared planes, not raw-only. catalog digest --prepared
  has no raw mode, so the deployer's digest-then-apply sequence cannot express
  the distinction at the boundary where it fails. An absent empty directory also
  carries no information the CAS does not already cover, so a raw-only carve-out
  would add a mode without adding a guarantee.
• Alternative rejected: re-materializing workspace facts in the raw snapshot.
  A raw preimage exists precisely because the declarations may be invalid, so it
  cannot know which directories are workspace facts — that is why fix(catalog): exclude workspaces from raw preimages #425 excluded
  them structurally.

Verification

New regression test reproduces the deployer's shape end to end (raw snapshot of
an invalid live plane → valid declaration bytes written into it → raw apply),
and asserts the fact is republished while live runtime content survives.

Negative control, same test with the source fix reverted:

thread 'raw_preimage_repairs_a_catalog_whose_declared_workspace_fact_is_runtime_only'
  panicked at tests/catalog_apply.rs:1932:5:
Error: inspect prepared workspace fact /tmp/.../agents/host/worker/.workspace

Caused by:
    No such file or directory (os error 2)

With the fix:

$ nix develop -c cargo test --test catalog_apply raw_preimage
running 8 tests
test raw_preimage_rejects_hard_linked_declarations ... ok
test raw_preimage_treats_the_live_envelope_and_pty_root_as_bytes ... ok
test raw_preimage_repairs_a_catalog_whose_declared_workspace_fact_is_runtime_only ... ok
test raw_preimage_migrates_legacy_argv_profile_to_a_component_catalog ... ok
test raw_preimage_repairs_an_invalid_catalog_and_preserves_mutable_state ... ok
test raw_preimage_accepts_valid_bytes_and_wrong_cas_preserves_declarations ... ok
test raw_preimage_repairs_catalogs_with_unadmitted_profile_modules ... ok
test raw_preimage_resume_uses_the_durable_validated_stage ... ok
test result: ok. 8 passed; 0 failed; 52 filtered out

Whole-file comparison against pristine main at fbd1ff6, same machine, same
command (nix develop -c cargo test --test catalog_apply):

main (fbd1ff6)   45 passed; 14 failed
this branch      46 passed; 14 failed

The 14 failures are identical in name on both sides — this change adds no
failure and fixes none of them. See Concerns.

Complexity

No new abstraction. One admission predicate goes from "must exist" to "must be a
real empty directory if it exists", plus the spec sentence that stated the old
rule.

Concerns

• catalog_apply has 14 pre-existing failures on main, unchanged here. Two
  shapes: several time out waiting for a helper's ready file (this machine was
  under load average ~90), and
  workspace_facts_are_empty_in_prepared_admitted_against_live_and_never_applied
  fails at its final apply with
  [root-count]: host 'host' must declare exactly one root agent; found 2 — its
  fixture declares two unsupervised agents. Reproduced with this branch's changes
  reverted, so it is not from this PR, and repairing that fixture is out of scope
  here. Worth a follow-up: it hides real breakage in exactly this file.
• That same test is edited here, minimally: its "missing prepared workspace fact
  is rejected" case asserted the contract this PR removes, so it is gone. The
  "nonempty is rejected" case stays.

Friction & bottlenecks

• The failure surfaces as inspect prepared workspace fact — an I/O context
  string on a NotFound — which reads like a permissions or path problem rather
  than "this admission rule is unsatisfiable for raw preimages". A rule that
  cannot be satisfied by a supported input shape deserves its own message.
• The pre-existing red suite meant the first signal of this bug came from a
  downstream Nix check rather than from st2's own tests.

Follow-ups

• Repair the workspace_facts_... fixture (two root agents) and get
  catalog_apply green so regressions in it are visible.

References

Refs #425, #424

Posted on behalf of @schickling

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agent_identity	dev3.direct.omp.quzh99p3
session	dev3.quzh99p3
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.1.2
agent_runtime	OMP 18.1.2
tooling_profile	dotfiles@9323c3a