feat(parsers): add agent-trace.dev v1 sidecar parser

Adds support for the open, vendor-neutral agent-trace.dev v1 spec
(https://agent-trace.dev/) as a third parser alongside the existing
Claude Code and Codex/Copilot native log parsers.

Why
---

ai-blame previously had to grow one bespoke parser per agent. The
agent-trace.dev v1 spec is an open, agent-agnostic JSON sidecar format
designed exactly for this attribution use case, and any producer of
the spec (first-party emitter or third-party exporter from native
logs) is unlocked at once.

What
----

- New `src/parsers/agent_trace.rs` reads `.agent-trace/*.json` records
  conforming to the v1 schema and emits one `EditRecord` per
  `files[].conversations[].ranges[]` entry.
- Registered in `ParserRegistry::new()` first so the unambiguous
  `.json` + spec-shaped discriminator wins before the `.jsonl`
  parsers see the file.
- `get_agent_trace_dirs()` discovers `<cwd>/.agent-trace/` and
  `~/.agent-trace/` automatically and feeds them into
  `get_all_trace_dirs()`.
- Smoke-tested end-to-end against 21 real spec records: 126 edits
  across 41 files extracted via `ai-blame stats` / `report`.

Limitations
-----------

agent-trace.dev records carry attribution ranges + `content_hash`
(spec §6.3) rather than raw `old_string` / `new_string` patches, so
`stats` / `timeline` / `report` / `transcript` work fully but
`blame` / `annotate` cannot perform the same patch-walk
reconstruction as the native parsers. Documented in the parser module
docstring and in the README.

Tests
-----

7 new unit tests in `parsers::agent_trace::tests` covering parse,
filter, fallback, can_parse acceptance/rejection, and
`collect_trace_files` extension filtering. Full `cargo test` is
green (22 passing); `cargo fmt --check` and `cargo clippy` clean.

Amp-Thread-ID: https://ampcode.com/threads/T-019e0ee7-99b6-72e7-a9eb-e71bba013ceb
Co-authored-by: Amp <amp@ampcode.com>

Author: joahg

README.md

@@ -215,11 +215,30 @@ rules:
 
 ## Supported Agents
 
-| Agent | Status |
-|-------|--------|
-| Claude Code | ✅ Supported |
-| OpenAI Codex / GitHub Copilot | ✅ Supported |
-| Others | PRs welcome! |
+| Agent | Status | Format |
+|-------|--------|--------|
+| Claude Code | ✅ Supported | Native `.jsonl` traces under `~/.claude/` |
+| OpenAI Codex / GitHub Copilot | ✅ Supported | Native `.jsonl` traces (incl. CLI ghost snapshots) |
+| Amp, Cursor, Goose, Cline, Kimi, … | ✅ Supported via [agent-trace.dev](https://agent-trace.dev/) | `.agent-trace/*.json` v1 sidecars (any producer) |
+| Others | PRs welcome! | |
+
+### agent-trace.dev v1
+
+ai-blame reads the open, vendor-neutral
+[agent-trace.dev v1 spec](https://agent-trace.dev/) from
+`<repo>/.agent-trace/*.json` (and `~/.agent-trace/`). Any tool that
+emits the spec — whether as a first-party feature or via a third-party
+exporter that converts an agent's native logs — is automatically
+supported.
+
+> agent-trace.dev records carry attribution ranges + `content_hash`
+> rather than raw `old_string` / `new_string` patches, so the `stats`,
+> `timeline`, `report`, and `transcript` commands work fully against
+> them; `blame` / `annotate` will mark touched lines with metadata but
+> cannot perform the same patch-walk reconstruction as the native Claude
+> / Codex parsers. See
+> [`src/parsers/agent_trace.rs`](src/parsers/agent_trace.rs) for the
+> mapping details.
 
 ## Differences from Python Version
 

src/extractor.rs

@@ -155,9 +155,33 @@ pub fn get_codex_trace_dirs() -> Vec<PathBuf> {
     dirs
 }
 
+/// Locate agent-trace.dev v1 sidecar directories. The spec is storage-agnostic
+/// but the conventional layout is `<repo>/.agent-trace/*.json`, so we look in
+/// the project root and the user's home as fallbacks.
+pub fn get_agent_trace_dirs() -> Vec<PathBuf> {
+    let mut dirs = Vec::new();
+
+    if let Ok(cwd) = std::env::current_dir() {
+        let local = cwd.join(".agent-trace");
+        if local.exists() {
+            dirs.push(local);
+        }
+    }
+
+    if let Some(home) = dirs::home_dir() {
+        let user_dir = home.join(".agent-trace");
+        if user_dir.exists() {
+            dirs.push(user_dir);
+        }
+    }
+
+    dirs
+}
+
 pub fn get_all_trace_dirs(claude_dir: &Path) -> Vec<PathBuf> {
     let mut dirs = vec![claude_dir.to_path_buf()];
     dirs.extend(get_codex_trace_dirs());
+    dirs.extend(get_agent_trace_dirs());
     dirs
 }