#22654: fix(signal): join debounced messages with actual newline instead of literal \\n

## Summary The Signal debounce flush handler joins buffered message texts with a literal two-character string `\n` instead of an actual newline character. Users receiving debounced messages see `hello\nworld` as-is, rather than messages separated by line breaks. ## Bug ```ts // src/signal/monitor/event-handler.ts, line 304 const combinedText = entries .map((entry) => entry.bodyText) .filter(Boolean) .join("\\n"); // ← escaped backslash-n: produces literal "\n" ``` In JavaScript, `"\\n"` is a two-character string (backslash + n), not a newline. When the debounce timer fires with multiple queued messages, they are concatenated with the literal text `\n` instead of a line break. ## Fix ```ts .join("\n"); // ← actual newline character ``` ## Test plan - [x] `npx oxfmt --check src/signal/monitor/event-handler.ts` — pass - [x] `npx oxlint src/signal/monitor/event-handler.ts` — 0 warnings, 0 errors - [x] `npx vitest run src/signal/monitor/event-handler.inbound-contract.test.ts --config vitest.unit.config.ts` — 2 tests pass 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes Signal debounced messages to use actual newlines instead of literal `\n` characters when combining multiple messages. - Changed `.join("\\n")` to `.join("\n")` in the debounce flush handler (line 304) - Aligns with Telegram and Discord implementations, which already use actual newlines - Follows repository guideline from `AGENTS.md`: "never embed `\\n`" for message content <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - Single-character fix that corrects an obvious bug (escaped backslash-n to actual newline). The change matches the pattern used in other channel handlers (Telegram, Discord) and follows the repository's style guidelines. Tests pass and the fix is clearly correct. - No files require special attention _{Last reviewed commit: 9cbc957} <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->