Summary

• Adds a before_agent_reply plugin hook that fires after slash commands but before the LLM agent runs
• Plugins can return { handled: true, reply } to short-circuit agent processing (forms, wizards, approval gates, etc. as plugins without touching core)
• Uses the runClaimingHook pattern (sequential by priority, first { handled: true } wins) — same pattern as inbound_claim
• Populates full PluginHookAgentContext including trigger, channelId, messageProvider

Motivation

Per VISION.md, core stays lean and optional capability should ship as plugins. Right now there's no way for a plugin to intercept an inbound message and return a synthetic reply before the LLM runs — anything that needs pre-LLM interception has to modify core. This hook fills that gap.

Closes #8807.

Design

Hook name: before_agent_reply

When it fires: After handleInlineActions returns kind: "continue", before stageSandboxMedia / runPreparedReply. This means /help and other slash commands still work normally, even during a plugin dialog.

Event type:

{ cleanedBody: string }  // final user message heading to LLM

Result type (claiming pattern):

{
  handled: boolean;      // true = claim this message, short-circuit the LLM
  reply?: ReplyPayload;  // synthetic reply (omit to silently swallow)
  reason?: string;       // for logging/debugging
}

Context: Full PluginHookAgentContext (agentId, sessionKey, sessionId, workspaceDir, messageProvider, trigger, channelId).

Execution: runClaimingHook — async, sequential by priority (highest first). First handler to return { handled: true } wins; remaining handlers are not called. When handled: true without reply, the message is swallowed via SILENT_REPLY_TOKEN.

Changes

Test plan

• pnpm test -- src/plugins/hooks.before-agent-reply.test.ts — all 8 tests pass
• pnpm test -- src/plugins/hooks — all 27 existing hook tests unaffected
• pnpm test -- src/auto-reply/reply/get-reply — existing get-reply tests pass
• pnpm oxlint / pnpm format — clean
• pnpm tsgo — no new type errors (pre-existing upstream errors only)
• git diff upstream/main --stat — exactly 4 files, no unrelated changes, no deletions of inbound_claim code