feat: add thread lifecycle contributor hooks

[jif-oai]

Why

Extensions that need thread-scoped state currently only get a start-time callback. That is enough for seeding stores, but it leaves the host without a shared extension seam for later thread rehydrate and flush work as thread ownership evolves. This PR turns that start-only seam into a host-owned thread lifecycle contributor contract so extension-private state can stay behind the extension API instead of leaking extra orchestration through core.

What changed

• Replaced ThreadStartContributor with ThreadLifecycleContributor and added typed lifecycle inputs for thread start, resume, and stop. The contract lives in contributors/thread_lifecycle.rs.
• Kept the existing start-time behavior intact by routing session construction through on_thread_start.
• Invoked on_thread_stop during session shutdown before thread-scoped extension state is dropped, while isolating contributor failures behind warning logs.
• Migrated git-attribution and guardian onto the lifecycle registration path.
• Renamed the extension registry plumbing from start-specific contributors to lifecycle-specific contributors.

Notes

on_thread_resume is introduced at the API boundary here so extensions can target the final lifecycle shape; host resume dispatch can be wired where that runtime path is finalized.

[jif-oai]

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[chatgpt-codex-connector]

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