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

README.md

@@ -0,0 +1,46 @@
+# Transcribe
+
+Officer's speech-to-text, talking to a self-hosted whisper.cpp server.
+
+Transcribe does not run Whisper and ships no model. You point it at a whisper.cpp `/inference` server
+you already run and it forwards to it, tuned for one person talking into a phone: detect the spoken
+language, then transcribe faithfully in it — never translated, never guessed at in English when the
+audio is not.
+
+## What you get
+
+- **`POST /api/transcribe/transcribe`** — send an audio file, get back its text and the language Whisper
+  detected. Two-pass under the hood (detect, then transcribe in that language) so a low-confidence guess
+  never quietly defaults to English.
+- **A file-browser task.** Right-click an `.mp3`, `.wav` or `.m4a` and transcribe it without a model
+  turn — the task is a plain script that calls this plugin's own route and files the result next to the
+  original.
+- **A skill**, so an agent asked to transcribe something knows the route, the shape, and what to say when
+  nothing is configured, without being told.
+
+## What you need
+
+**A whisper.cpp server you already run**, reachable from this machine, answering `POST /inference` —
+the same container the platform used to talk to directly. Nothing else. No system packages, no host
+binaries; the sidecar reaches it with `fetch`.
+
+## Setup
+
+```
+PUT /api/transcribe/config
+{ "url": "http://192.168.1.50:64201" }
+```
+
+Validated against the server before it is stored — a wrong URL is a 400 with the reason, not a saved
+value that fails on the first real transcription.
+
+## What this plugin deliberately does not have
+
+- **No dashboard route, no dock tile.** There is nothing to look at — this is reached through the file
+  browser's task menu and by an agent that has read the skill, not through a screen. Shipping no `web/`
+  is a supported shape, not an oversight.
+- **No streaming.** One file in, one answer back. If you want a live microphone, that is a different
+  feature built on top of this one, not something this plugin does today.
+- **No translation.** It transcribes in whatever language was spoken. Whisper's own translate mode is
+  English-only, so honouring "translate into <language>" for anything else would mean silently
+  handing back a rough English rendering of, say, Portuguese audio — worse than refusing.