#15577: feat(hooks): add message:preprocessed hook event

## Summary Adds a `message_preprocessed` plugin hook event that fires **after** media understanding (transcription, image descriptions) and link understanding (summaries, previews), but **before** agent processing begins. This completes the three-event message lifecycle: 1. `message:received` — raw inbound message 2. **`message:preprocessed`** — enriched with transcripts, media descriptions, link summaries *(this PR)* 3. `message:sent` — outbound reply ### Why? Plugins that need enriched content (e.g., memory systems, analytics, logging) currently have to either re-process media themselves or hook into `message:received` which only has raw content. The `message:preprocessed` hook gives plugins access to the fully enriched message context with zero extra work. ### What's included - `PluginHookMessagePreprocessedEvent` type with `content`, `rawContent`, `transcript`, and metadata - `runMessagePreprocessed()` on the hook runner - Internal hook event (`message:preprocessed`) for extension systems - Hook fires in `getReplyFromConfig()` after `applyMediaUnderstanding()` and `applyLinkUnderstanding()` ### Builds on PR #14196 which added `message:received` and `message:sent` hooks (shipped in 2026.2.12). Addresses discussion #14539 (three-event lifecycle proposal). <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR introduces a new plugin hook event, `message_preprocessed`, intended to fire after media/link understanding but before agent processing. It updates the plugin hook type map (`src/plugins/types.ts`) and exposes a runner method (`src/plugins/hooks.ts`), then triggers the hook from `getReplyFromConfig` in `src/auto-reply/reply/get-reply.ts` (alongside an internal `message:preprocessed` hook event). The main integration point is `getReplyFromConfig`, which now constructs a `PluginHookMessagePreprocessedEvent` and dispatches it via the global hook runner before continuing to command authorization/session/agent logic. <h3>Confidence Score: 3/5</h3> - Mostly safe to merge, but plugin hook payload correctness should be fixed to avoid breaking plugin expectations. - The change is localized and adds a new hook without altering core agent behavior, but the emitted `message_preprocessed` payload currently has inconsistencies vs existing message hooks (content/rawContent source/normalization) and can produce invalid timestamps (NaN / mishandling 0). These are user-facing API contract issues for plugin authors and should be corrected before merging. - src/auto-reply/reply/get-reply.ts _{Last reviewed commit: aab320b} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->