fix(plugin): add context.Context to HookFunc for cooperative cancellation

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Summary

• Changes HookFunc signature from func(payload interface{}) (interface{}, error) to func(ctx context.Context, payload interface{}) (interface{}, error)
• Run() passes context.Background() to each hook
• RunWithTimeout() uses context.WithTimeout() and passes the derived context, enabling cooperative cancellation
• Implements the full hook registry (was stubs on main): registration, chaining, timeout with warnings
• Updates PLAN.md and IMPLEMENTATION.md to document the context-aware hook API

Closes #13

Test plan

• All 24 hooks tests pass with updated context.Context signatures
• Timeout tests verify context-based cancellation behavior
• Registry/node/wasm tests unaffected (separate stub implementations)

🤖 Generated with Claude Code

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Code Review: fix(plugin): add context.Context to HookFunc for cooperative cancellation

Verdict: Approve with merge sequencing note ✅

Correctly addresses the goroutine leak I flagged on PR #8. The HookFunc signature now carries a context.Context with the timeout deadline, enabling cooperative cancellation.

Check	Result
Hooks tests	24/24 pass
Other plugin tests	35 fail (pre-existing on main: 0/59 pass without Phase 5)
go vet	Clean
Regressions	None

Implementation review:

• Run() correctly passes context.Background() — no timeout for direct execution
• RunWithTimeout() creates context.WithTimeout() per hook, passes the derived context, cancels in both branches (success and timeout)
• Timeout behavior preserved: pre-hook payload kept on timeout, warning logged, build continues
• Hook functions can now check ctx.Done() for cooperative cancellation

Test changes: All 24 test modifications are mechanical — adding _ context.Context as the first parameter to match the new signature. Test assertions are unchanged. The hooks are intentionally context-unaware in tests to verify the timeout safety net works even without cooperation.

Merge sequencing concern: PR #8 (Phase 5) also implements hooks.go from stubs with the old HookFunc signature. When both PRs merge to main, they will conflict on hooks.go and hooks_test.go. Recommendation: merge this PR first, then rebase PR #8 to adopt the context.Context signature.

PLAN.md/IMPLEMENTATION.md updated with the context-aware hook API documentation. Clean fix.