feat(plugin): add session.stopping hook for loop continuation

[orenbaldinger]

Issue for this PR

Closes #16626

Type of change

• New feature

What does this PR do?

Adds an awaited session.stopping plugin hook in the Hooks interface to allow external plugins to evaluate session state and optionally inject a continuation message before the prompt loop breaks.

Key semantics:

• Awaited Lifecycle: Dispatched in SessionPrompt.run immediately prior to natural loop exit.
• Fail-Closed Default: Output defaults to stop: true. If a plugin listener throws or rejects, execution stops cleanly (stop: true, message: undefined).
• Sticky Veto: Multiple listeners run in registration order. If any listener sets stop: true, subsequent listeners cannot force continuation.
• Defense-in-Depth Cap: Core enforces SESSION_STOPPING_REENTRY_CAP = 3 to prevent runaway infinite loops from misconfigured plugins.
• Cleanup: In-memory continuation state is pruned on session.deleted.

How did you verify your code works?

• bun test packages/opencode/test/plugin/session-stopping.test.ts (listener ordering, error isolation, fail-closed behavior)
• bun test --timeout 30000 packages/opencode/test/plugin/session-stopping-exit-matrix.test.ts (compaction stops, provider/tool error non-natural exits, nested sessions, concurrent loop callers)
• bun turbo typecheck across all workspace packages

Screenshots / recordings

N/A — backend lifecycle hook change without UI modifications.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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