Hook bus kernel: actions + filters with priority ordering

[tayebmokni]

Summary

Implement the hook bus (actions + filters) that the comment, post-transition, permalink, and meta subsystems use to fan events out to plugin handlers. WASM dispatch is owned by doc 02 — this issue ships the in-process kernel.

Design reference

• docs/00-architecture-overview.md §3.1 (Hook model: actions + filters)
• docs/01-core-cms.md §5.3 (hook on post.transitioned)
• docs/01-core-cms.md §6.3 (pre_comment filter)
• docs/01-core-cms.md §7.4 (permalink.changed hook)

Acceptance criteria

• internal/hooks package exports Bus with RegisterAction(name, prio, fn), RegisterFilter(name, prio, fn), DoAction(name, args), ApplyFilters(name, value, args) value
• Priority ordering: lower runs first; stable for equal priorities
• Filters chain: each filter receives the result of the previous one
• Action panics are caught, logged, and do not crash the request
• Bus is goroutine-safe
• Hooks fire in the documented places: post.transitioned, pre_comment, comment.transitioned, permalink.changed
• WASM-side dispatch is a NO-OP in this issue (interface defined for future)
• Unit tests cover priority ordering, panic isolation, filter chaining

Dependencies

#22

Complexity

M