feat(telegram): add outbound sanitizer leak corpus and docs

- Add leak corpus test cases (tests/data/telegram_leak_cases.json)
- Add sanitizer documentation (docs/telegram-sanitizer.md)
- Block internal diagnostics from reaching users
- Strip wrapper artifacts from LLM output
- Static response for unknown slash commands

docs/telegram-sanitizer.md

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+# Telegram Outbound Sanitizer
+
+This document describes the Telegram outbound sanitizer behavior for preventing internal diagnostics and wrapper artifacts from reaching end users.
+
+## Overview
+
+The sanitizer intercepts Telegram outbound messages and:
+
+1. Strips wrapper artifacts (`<reply>`, `<NO_REPLY>`, `<tool_schema>`, etc.)
+2. Drops internal diagnostics (error codes, run IDs, gateway details)
+3. Returns static responses for unknown slash commands
+
+## Marker Families
+
+Static checks verify these marker families:
+
+- `OPENCLAW_TELEGRAM_OUTBOUND_SANITIZER`
+- `OPENCLAW_TELEGRAM_INTERNAL_ERROR_SUPPRESSOR`
+
+## Leakage Patterns Blocked
+
+### Tool/Runtime Leakage
+
+- `tool call validation failed`
+- `not in request.tools`
+- `sessions_send` templates / `function_call`
+- `Run ID`, `Status: error`, gateway timeout/connect details
+
+### Media/Tool Scaffolding
+
+- `MEDIA:`/`.MEDIA:` leak lines
+- TTS scaffolding text
+
+### Sentinel/Garbage Markers
+
+- `NO_CONTEXT`, `NOCONTENT`, `NO_MESSAGE_CONTENT_HERE`
+- `NO_DATA_FOUND`, `NO_API_KEY`
+
+## Enforced Behavior
+
+1. **Unknown slash commands** → static text response
+2. **Unknown slash commands** → does NOT call LLM
+3. **Telegram output** → never emits tool diagnostics/internal runtime details
+4. **Optional debug override** → owner-only with `TELEGRAM_DEBUG=true`
+
+## Verification
+
+Run the leak corpus tests:
+
+```bash
+# Run leak case corpus validation
+pnpm test src/telegram/sanitizer.test.ts
+
+# Manual smoke check
+# In any Telegram chat: /unknown_command
+# Expected: "Unknown command. Use /help."
+```
+
+## Test Corpus
+
+The test corpus at `tests/data/telegram_leak_cases.json` contains:
+
+- `expect: "allow"` - Messages that should pass through
+- `expect: "drop"` - Messages that should be blocked
+- `expect: "strip_wrapper"` - Messages that need wrapper removal
+
+## Implementation Notes
+
+- Sanitization runs after LLM response, before Telegram API send
+- Empty payloads after sanitization return fallback message
+- Return shape `{ queuedFinal, counts }` is preserved for caller safety