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.

.env.example

@@ -56,6 +56,12 @@ UNSPLASH_ACCESS_KEY=         # Unsplash stock images (free developer key)
 
 # --- Analysis ---
 HF_TOKEN=                    # HuggingFace token — enables speaker diarization in transcriber
+# Speech-to-text: optional Azure AI Speech (Fast Transcription). When set, the
+# agent prefers azure_stt for cloud STT; the local faster-whisper transcriber
+# remains the default offline path.
+AZURE_SPEECH_KEY=            # Azure AI Speech resource key ('Keys and Endpoint' page)
+AZURE_SPEECH_REGION=         # Speech resource region, e.g. eastus
+# AZURE_SPEECH_ENDPOINT=     # Optional: full custom endpoint URL (overrides region)
 
 # --- Avatar (local installs) ---
 # WAV2LIP_PATH=              # Path to cloned Wav2Lip repo (for lip sync)