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

skills/transcribe/SKILL.md

@@ -0,0 +1,69 @@
+---
+name: Transcribe
+description: Use when you need to turn a recording into text — a voice memo, a meeting, dictation on disk. Carries the route, the request shape, and what to do when nothing is configured.
+version: 1
+---
+
+Officer's speech-to-text runs entirely inside the `transcribe` plugin, over your own whisper.cpp server.
+Nothing else on the platform reads audio for you — a request that mentions a recording, a voice memo, a
+call, a lecture, means this plugin.
+
+**There is no model turn here.** The plugin does the whole job itself: detect the spoken language, then
+transcribe faithfully in it. Do not narrate the audio yourself, do not summarize it in place of
+transcribing, and do not chain a translation step — the plugin is already faithful to the source language,
+and Whisper's translate mode is English-only, so anything past it would be a rough loss rather than a
+feature.
+
+## Is it set up?
+
+```
+GET /api/transcribe/config
+→ { "configured": true,  "url": "http://192.168.1.50:64201" }
+→ { "configured": false, "url": null }
+```
+
+If `configured` is `false`, say so plainly and stop — "speech-to-text isn't set up yet; point it at a
+whisper.cpp server with `PUT /api/transcribe/config`" — rather than attempting the transcription and
+reporting a generic failure. Setting it up is a one-time, deliberate act (it validates the URL against the
+server before storing it), not something to do silently on the caller's behalf without being asked.
+
+```
+PUT /api/transcribe/config
+{ "url": "http://192.168.1.50:64201" }
+→ { "configured": true, "url": "http://192.168.1.50:64201" }
+→ 400 { "error": "…" }   — url missing, malformed, or the server did not answer
+```
+
+## Transcribing
+
+```
+POST /api/transcribe/transcribe
+multipart/form-data:
+  file             — the audio file (mp3, wav, m4a, whatever whisper.cpp accepts)
+  spokenLanguages  — zero or more, e.g. spokenLanguages=en&spokenLanguages=pt
+
+→ { "text": "…", "detectedLanguage": "en" }
+→ 400 { "error": "Whisper not configured — PUT /api/transcribe/config first" }
+→ 502 { "error": "…" }   — the whisper.cpp server rejected or failed the request
+```
+
+`spokenLanguages` only matters when Whisper's own detection is genuinely unsure — it breaks the tie
+toward a language you already know the speaker uses. Pass what you know; pass nothing and detection still
+works, it is just less informed on an ambiguous clip.
+
+**This can take minutes** on a long recording — whisper.cpp runs at roughly 8x realtime, so a 40-minute
+memo is a couple of minutes of waiting, not a couple of seconds. Do not treat a slow response as a failure
+and retry; wait for it.
+
+## The file-browser task
+
+`tasks/transcribe-audio-file` in this plugin ships a right-click task for `.mp3`/`.wav`/`.m4a` files that
+calls this same route and files the result next to the original — a **script** task, deliberately, with no
+model turn per audio file. If you are asked to transcribe something already sitting in the file browser,
+prefer pointing the user at that over doing it yourself turn-by-turn.
+
+## What this is not
+
+Not a live dictation microphone — there is no streaming endpoint, only a file upload. Not a translator —
+it transcribes in whatever language was spoken, never into a different one. And it holds no memory of past
+transcriptions; every call is independent.