[P2-8] Plugin lifecycle hooks: pre/post tool, pre/post LLM, session start/end (Rust HookRegistry + SDK plugins.on)

[justrach]

Parent: #34
Priority: P2 (extensibility — gateway/cron/memd all rely on hooks)
Branch base: release/0.1.53

Why

Hermes' plugin contract registers six lifecycle hooks: pre_tool_call, post_tool_call, pre_llm_call, post_llm_call, on_session_start, on_session_end. graff-agent's gateway uses on_session_start/end for chat mapping; memd uses post_tool_call to detect failure patterns; cron uses on_session_end to log cron-run outcomes.

Scope

Rust side

• forge_domain::Hook trait + a HookRegistry on ForgeAPI.
• Six hooks fired from forge_app:
  • pre_tool_call(tool_call) — can mutate or veto (return HookOutcome::Veto(reason)).
  • post_tool_call(tool_call, result).
  • pre_llm_call(request).
  • post_llm_call(response).
  • on_session_start(cid).
  • on_session_end(cid).
• Hooks are async, invoked sequentially per registration order, errors logged but don't crash the agent.

SDK side

• graff.plugins.on(event, handler) — typed events.
• TS:

  graff.plugins.on("preToolCall", async (tc) => { /* ... */ });

Acceptance

• Each hook fires at the documented point
• Veto from preToolCall skips the tool and surfaces an error to the agent
• Multiple handlers registered for one event run in order
• Crash in a handler logs but doesn't kill the conversation