Fix: atomic plugins.toml writes — registry race made plugin subcommands vanish

[0xLeif]

Summary

• save_registry wrote plugins.toml with fs::write (truncate-in-place). A concurrent fledge invocation reading the registry mid-rewrite parsed partial TOML, and because dispatch loads the registry with .ok()?, every registered plugin command silently resolved to unrecognized subcommand.
• Observed in the wild on merlin's CI: the trust-gate's fledge run augur-gate failed with unrecognized subcommand 'augur' three times in two days — every time a plugin registration (merlin init/update, fledge plugins install) overlapped the gate. The plugin was perfectly installed each time; the red builds were pure race.
• Fix: write-then-rename in save_registry (same-directory rename is atomic on POSIX — readers see the old registry or the new one, never a torn one), plus a one-shot 50ms retry in load_registry for rewrites by older non-atomic writers, plus a stderr warning when the registry is persistently unreadable so the failure is diagnosable instead of masquerading as a missing plugin.

Test plan

• New regression test: 400 reads against a continuously rewriting writer thread — zero torn reads (fails reliably against the old fs::write implementation)
• New test: corrupt registry surfaces an error rather than parsing as empty
• fledge lanes run ci — all 6 steps green

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces atomic registry saving using a write-then-rename pattern to prevent concurrent readers from observing a truncated plugins.toml file. It also adds a retry mechanism when loading the registry to handle potential concurrent writes from older versions, along with corresponding unit tests. The review feedback suggests cleaning up the temporary file if the write or rename operation fails to prevent leaking temporary files.

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[github-actions]

❌ Corvin says...

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"Even the dumpster of code seems empty today."

CI Summary

Check	Status
Dependency Audit	✅ Passed
Integration (3 OS)	❌ failure
Lint (fmt + clippy)	✅ Passed
Spec Validation	✅ Passed
Tests (3 OS)	✅ Passed

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Superseded by updated review.

[github-actions]

✅ Corvin says...

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"Looking sharp! Like a beak should be."

CI Summary

Check	Status
Dependency Audit	✅ Passed
Integration (3 OS)	✅ Passed
Lint (fmt + clippy)	✅ Passed
Spec Validation	✅ Passed
Tests (3 OS)	✅ Passed

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[github-actions]

✅ Corvin says...

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    <(^\  .oO(Caw! ^v^)
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"Caw! Found a shiny new spec!"

CI Summary

Check	Status
Dependency Audit	✅ Passed
Integration (3 OS)	✅ Passed
Lint (fmt + clippy)	✅ Passed
Spec Validation	✅ Passed
Tests (3 OS)	✅ Passed

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