feat: add AI Signals Landscape report — wide-table view of all trending signals

[Copilot]

Users wanted a consolidated "wide view" of all trending AI signals in a single artifact, rather than the narrative-per-category format of the existing trending report.

New report type: ai-signals / ai-signals-en

Generates a single wide Markdown table from the same TrendingData used by the trending report, with one row per AI project sorted by today's momentum:

| Project | Category | ⭐ Stars | 📈 Today | Stack | Signal |
|---------|----------|---------|----------|-------|--------|
| bytedance/deer-flow | 🤖 Agents | 0 | +3,787 | Python | ... |

Followed by a Signal Synthesis section (3–5 sentences on dominant themes and breakout stars).

Changes

• src/prompts-data.ts — buildSignalsPrompt(): instructs the LLM to deduplicate both sources, sort by today's stars, and emit the table + synthesis only
• src/i18n.ts — SIGNALS_REPORT constant; signals/signals-en entries in ISSUE_LABELS, REPORT_LABELS, NOTIFY_LABELS
• src/report-savers.ts — saveSignalsReport(): LLM call → file save → optional GitHub issue, mirrors saveTrendingReport structure
• src/index.ts — wired into parallel save phase (zh + en); ai-signals files included in highlights scan
• src/github.ts — label colors: signals #0ea5e9, signals-en #38bdf8
• src/generate-manifest.ts — ai-signals / ai-signals-en added to REPORT_FILES
• index.html — ai-signals / ai-signals-en added to LABELS for sidebar