docs: update beads.jsonl references to issues.jsonl

Fixes GH#409 - Documentation inconsistently referred to beads.jsonl
as the canonical filename when issues.jsonl has been the default
since v0.25.1 (bd-6xd).

Updated files:
- docs/ARCHITECTURE.md
- docs/CLI_REFERENCE.md
- docs/PROTECTED_BRANCHES.md
- docs/ADVANCED.md
- docs/GIT_INTEGRATION.md
- docs/MULTI_REPO_AGENTS.md
- docs/MULTI_REPO_MIGRATION.md
- docs/TROUBLESHOOTING.md
- examples/*/README.md
- skills/beads/references/CLI_REFERENCE.md

Note: beads.jsonl is still supported for backward compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/GIT_INTEGRATION.md

@@ -595,7 +595,7 @@ git config merge.beads.driver
 bd init --skip-db  # Only reconfigure git, don't touch database
 
 # Verify .gitattributes
-grep "beads.jsonl" .gitattributes
+grep "issues.jsonl" .gitattributes
 # Expected: .beads/issues.jsonl merge=beads
 ```