test(mcp): store template seeding always fails and falls back to full init

[ScriptedAlchemy]

Symptom

crates/tracedecay/tests/mcp_suite/fixture.rs::seed_store rewrites config.json in the seeded project data dir, but project stores no longer contain config.json (configuration lives in the store databases; the only remaining config.json references are the retired checkout layout list and a test that writes it as a stale input). rewrite_json therefore returns NotFound after the whole template tree has already been copied into place, init_project_from_template* swallows the error (seed_store(..).is_ok()), and every caller falls back to TraceDecay::init on top of the partially seeded directory.

So the template never saves the init cost it exists for, and a stale template still breaks init: the copied store stays in place, which is how #2213 surfaced as ResetRequired instead of a clean rebuild.

Evidence

Calling seed_store directly on a freshly built template (while writing the test for #2213) on origin/master f1576d0:

thread 'fixture::template_recorded_at_an_older_git_correlation_version_is_rebuilt' panicked at crates/tracedecay/tests/mcp_suite/fixture.rs:562:66:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Expected

Seeding either rewrites only files the current store layout has and opens the seeded store, or the template is deleted if it no longer saves anything. A failed seed must not leave a partial store for the fallback init to trip over.

Found while fixing #2213; not changed there (that fix only keys the template on the schema versions).