docs: add Multi-IDE Configuration Sync pattern (v3.20.9)

Added comprehensive documentation for maintaining consistent AI instructions
across multiple coding tools (Claude Code, Cursor, Copilot, etc.).

Changes:
- guide/ai-ecosystem.md: New "Multi-IDE Configuration Sync" section (L1256-1329)
  - Problem statement with config file comparison table
  - Solution 1: Native @import for Claude Code (recommended)
  - Solution 2: Script-based generation for multi-IDE teams (bash example)
  - AGENTS.md support status clarification + workaround (symlink)
  - Compatibility matrix with external tool references
- machine-readable/reference.yaml: Added 2 index entries
  - ai_ecosystem_multi_ide_sync: guide/ai-ecosystem.md:1256
  - agents_md_support_status: guide/ai-ecosystem.md:1322
- CHANGELOG.md: Documented addition in [Unreleased] section
- VERSION: Bumped from 3.20.8 to 3.20.9 (patch)
- Auto-synced version across README, cheatsheet, ultimate-guide

Total: +91 insertions, -8 deletions (74 lines net content addition)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

machine-readable/reference.yaml

@@ -3,7 +3,7 @@
 # Source: guide/ultimate-guide.md
 # Purpose: Condensed index for LLMs to quickly answer user questions about Claude Code
 
-version: "3.20.8"
+version: "3.20.9"
 updated: "2026-02-01"
 
 # ════════════════════════════════════════════════════════════════
@@ -500,6 +500,8 @@ deep_dive:
   ai_ecosystem_goose: "guide/ai-ecosystem.md:1116"
   ai_ecosystem_goose_comparison: "guide/ai-ecosystem.md:1132"
   ai_ecosystem_context_packing: "guide/ai-ecosystem.md:1208"
+  ai_ecosystem_multi_ide_sync: "guide/ai-ecosystem.md:1256"
+  agents_md_support_status: "guide/ai-ecosystem.md:1322"
   # Architecture Diagrams as Context (advanced pattern) - Added 2026-01-25
   architecture_diagrams_input: "guide/ai-ecosystem.md:1379"
   architecture_diagrams_mcp_tools:
@@ -927,7 +929,7 @@ ecosystem:
       - "Cross-links modified → Update all 4 repos"
     history:
       - date: "2026-01-20"
-        event: "Code Landing sync v3.20.8, 66 templates, cross-links"
+        event: "Code Landing sync v3.20.9, 66 templates, cross-links"
         commit: "5b5ce62"
       - date: "2026-01-20"
         event: "Cowork Landing fix (paths, README, UI badges)"