extract speech-to-text into its own plugin

Moves platform STT out into a plugin that owns the whole whisper.cpp contract: sidecar,
config (a JSON file under DATA_PATH/transcribe/, validated against the server before
storing), a thin proxy router, a skill so an agent knows the contract without being told,
and a script-mode file-browser task (not agentic — no model turn per audio file).

sidecar/transcribe.ts and its test moved unchanged from src/servers/api/stt/ — every
constant in it has a reason attached and none of it was touched.

The platform-side removal (routes, dead useAudioRecording path, Settings screen, locale
keys) lands in a separate, uncommitted change in the platform repo for review.

Not yet reachable from the file browser: the platform does not load tasks from a plugin's
directory yet (skills/affordances/notifications do). Ships anyway per instructions.

Officer-Session: 656f5c85-e49a-4f1f-b9f3-042a04b4d043

tasks/transcribe-audio-file/TASK.md

@@ -0,0 +1,58 @@
+---
+name: Transcribe Audio File
+description: Transcribe an audio file to text over your own whisper.cpp server, no model turn per file.
+category: Media
+version: 1
+mode: script
+inline: false
+language: typescript
+tags:
+  - audio
+  - transcription
+skills:
+  - transcribe
+inputs:
+  - name: file_path
+    description: Path to the audio file to transcribe. Auto-filled from the file browser.
+    required: true
+trigger:
+  - type: file
+    extensions:
+      - mp3
+      - wav
+      - m4a
+---
+
+# Transcribe Audio File
+
+Sends the audio file to the `transcribe` plugin's own route — `POST /api/transcribe/transcribe` — and
+files the result next to the original.
+
+A **script** task, deliberately: the plugin already does its own language detection and transcription
+server-side, so there is nothing here worth a model turn for. Running one per audio file would be a
+model call spent re-deriving what the plugin already returns.
+
+## What it does
+
+1. Read the audio file at `file_path` and the caller's spoken-language preferences
+   (`GET /api/user/settings`).
+2. `POST` the file to `/api/transcribe/transcribe`. The plugin detects the language and transcribes
+   faithfully in it — see `plugins/transcribe/skills/transcribe/SKILL.md` for the contract.
+3. Create a sibling directory named `<date>_<slug>`, where `<date>` is today in `YYYYMMDD` and `<slug>`
+   is derived from the original file's own name (not a generated title — a script task gets no model
+   turn to ask for one).
+4. Move the original audio file into that directory and save the transcription beside it as a
+   Markdown file with the same base name.
+
+## Before it runs
+
+The plugin has to be configured — `PUT /api/transcribe/config` with a reachable whisper.cpp URL. If it
+is not, this fails with that told plainly rather than a generic error.
+
+## What it deliberately does not do
+
+The worked example this was adapted from (an agentic task) ran its own detect-language pass before
+transcribing, and generated a title from the transcription's contents. Both are dropped here: the
+detect-then-transcribe step is already what the plugin's route does internally, so repeating it would be
+redundant work against the same server. And a generated title needs a model turn, which is exactly what
+this task exists to avoid spending per file.