Remove sequential ID generation and SyncAllCounters (bd-c7af, bd-8e05, bd-4c74)

- Removed SyncAllCounters() and all call sites (already no-op with hash IDs)
- Removed AllocateNextID() and getNextIDForPrefix() - sequential ID generation
- Removed collision remapping logic in internal/storage/sqlite/collision.go
- Removed rename collision handling in internal/importer/importer.go
- Removed branch-merge example (collision resolution no longer needed)
- Updated EXTENDING.md to remove counter sync examples

These were all deprecated code paths for sequential IDs that are obsolete
with hash-based IDs. Hash ID collisions are handled by extending the hash,
not by remapping to new sequential IDs.

examples/README.md

@@ -9,7 +9,7 @@ This directory contains examples of how to integrate bd with AI agents and workf
 - **[markdown-to-jsonl/](markdown-to-jsonl/)** - Convert markdown planning docs to bd issues
 - **[github-import/](github-import/)** - Import issues from GitHub repositories
 - **[git-hooks/](git-hooks/)** - Pre-configured git hooks for automatic export/import
-- **[branch-merge/](branch-merge/)** - Branch merge workflow with collision resolution
+<!-- REMOVED (bd-4c74): branch-merge example - collision resolution no longer needed with hash IDs -->
 - **[claude-desktop-mcp/](claude-desktop-mcp/)** - MCP server for Claude Desktop integration
 - **[claude-code-skill/](claude-code-skill/)** - Claude Code skill for effective beads usage patterns
 
@@ -28,9 +28,7 @@ cd bash-agent
 cd git-hooks
 ./install.sh
 
-# Try branch merge collision resolution
-cd branch-merge
-./demo.sh
+# REMOVED (bd-4c74): branch-merge demo - hash IDs eliminate collision resolution
 ```
 
 ## Creating Your Own Agent