fix(router): escape Discord markdown delimiters instead of stripping

Previously formatOutbound stripped `*`, `_`, `~`, `|`, `` ` ``, `#`, `>`
from prose so they wouldn't trigger Discord formatting. That worked but
silently lost information — `**DONE**`, `STEP_DONE`, ~~strike~~ all
arrived in Discord with the source characters missing.

Switch to backslash-escape so the literal source text shows up while
still suppressing the formatting interpretation. Triple-backtick fenced
code blocks are still preserved verbatim.

Escape is non-idempotent (running it twice double-escapes backslashes),
so split the pipeline:

  - sanitizeForOutbound: strip internal tags + tool-call leaks + redact
    secrets. Use this for intermediate text that will pass through
    another formatOutbound call downstream (work-item storage, channel
    wrappers, session-command output).
  - formatOutbound: sanitize + neutralizeStrayMarkdown. Reserved for the
    single Discord-send boundary in channels/discord.ts.

Internal callers (message-turn-controller, session-commands, the
sendFormattedChannelMessage / sendFormattedTrackedChannelMessage /
editFormattedTrackedChannelMessage wrappers in index.ts, and the cron
reviewer-bot path) now use sanitizeForOutbound so the markdown escape
runs exactly once at the channel boundary.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/message-turn-controller.ts

@@ -4,7 +4,7 @@ import {
   getAgentOutputText,
 } from './agent-output.js';
 import { createScopedLogger, logger } from './logger.js';
-import { formatOutbound } from './router.js';
+import { sanitizeForOutbound } from './router.js';
 import { shouldResetSessionOnAgentFailure } from './session-recovery.js';
 import { TASK_STATUS_MESSAGE_PREFIX } from './task-watch-status.js';
 import { formatElapsedKorean } from './utils.js';
@@ -166,7 +166,10 @@ export class MessageTurnController {
     }
 
     const raw = getAgentOutputText(result);
-    const text = raw ? formatOutbound(raw) : null;
+    // Use sanitize (no markdown escape) — the Discord channel boundary
+    // applies the final escape pass, and double-escaping would produce
+    // visible backslash garbage.
+    const text = raw ? sanitizeForOutbound(raw) : null;
     const attachments = getAgentOutputAttachments(result);
 
     if (raw) {