feat(stt): add Azure AI Speech as an optional cloud speech-to-text provider

Add an Azure AI Speech transcription tool. It is opt-in: when
AZURE_SPEECH_KEY is configured the agent may prefer it for cloud STT,
while the local faster-whisper `transcriber` stays the default offline
path. Shared pipeline manifests are intentionally left unchanged, so no
default provider selection is altered for existing users.

- tools/analysis/azure_stt.py: new `azure_stt` tool (capability=analysis,
  provider=azure) calling the Fast Transcription REST API. The local file
  is uploaded via multipart and transcribed synchronously with word-level
  timestamps and optional diarization — no Blob storage or async polling.
  Output schema mirrors `transcriber` exactly, so it is a drop-in for
  `subtitle_gen` and other transcript consumers. Follows the existing
  provider-tool conventions (env-var status check, `_transcribe` helper,
  cost_usd/model on the result, fallback="transcriber").
- Auto-discovered by the registry; no registry or selector changes.
- tests/tools/test_azure_stt.py: contract, discovery, status, response
  mapping, execute guardrails, and a mocked-network success path (no live
  API calls).
- .agents/skills + .claude/skills: azure-speech-to-text Layer-3 skill.
- docs/PROVIDERS.md: Azure AI Speech setup, API notes, and pricing.
- .env.example, skills/INDEX.md, AGENT_GUIDE.md: document the optional
  cloud STT path alongside the default whisper transcriber.

AGENT_GUIDE.md

@@ -678,7 +678,8 @@ The `.agents/skills/` directory is large. When you're not coming in through a to
 | **Image generation** | `bfl-api`, `flux-best-practices` |
 | **Video generation** | `seedance-2-0` (preferred premium default — cinematic, trailer, multi-shot, synced audio, lip-sync), `gemini-omni` (conversational video editing, reference tags, timecoded beats), `ai-video-gen`, `ltx2` |
 | **Audio** | `elevenlabs`, `music`, `sound-effects`, `acestep`, `text-to-speech`, `setup-api-key` |
-| **Avatar / lip-sync** | `avatar-video`, `heygen`, `create-video`, `faceswap`, `video-translate`, `speech-to-text`, `agents` |
+| **Speech-to-text** | `speech-to-text` (whisper `transcriber` — default, offline), `azure-speech-to-text` (optional cloud STT — tool `azure_stt`, preferred when `AZURE_SPEECH_KEY` is set) |
+| **Avatar / lip-sync** | `avatar-video`, `heygen`, `create-video`, `faceswap`, `video-translate`, `agents` |
 | **Capture** | `playwright-recording` (browser flows), `ffmpeg` (post) |
 | **Visualization** | `beautiful-mermaid`, `d3-viz`, `manim-composer`, `manimce-best-practices`, `manimgl-best-practices` |
 | **Media editing** | `video-edit`, `video-download`, `video-understand`, `video-toolkit`, `visual-style` |