Implement BEADS_DIR environment variable (bd-e16b)

Add BEADS_DIR as a replacement for BEADS_DB to point to the .beads
directory instead of the database file directly.

Rationale:
- With --no-db mode, there's no .db file to point to
- The .beads directory is the logical unit (contains config.yaml, db
  files, jsonl files)
- More intuitive: point to the beads directory not the database file

Implementation:
- Add BEADS_DIR environment variable support to FindDatabasePath()
- Priority order: BEADS_DIR > BEADS_DB > auto-discovery
- Maintain backward compatibility with BEADS_DB (now deprecated)
- Update --no-db mode to respect BEADS_DIR
- Update MCP integration (config.py, bd_client.py)
- Update documentation to show BEADS_DIR as preferred method

Testing:
- Backward compatibility: BEADS_DB still works
- BEADS_DIR works with regular database mode
- BEADS_DIR works with --no-db mode
- Priority: BEADS_DIR takes precedence over BEADS_DB

Follow-up issues for refactoring:
- bd-efe8: Refactor path canonicalization into helper function
- bd-c362: Extract database search logic into helper function

Closes bd-e16b

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

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

examples/claude-code-skill/references/CLI_REFERENCE.md

@@ -371,13 +371,13 @@ Use when parsing programmatically or extracting specific fields.
 bd finds database in this order:
 
 1. `--db` flag: `bd ready --db /path/to/db.db`
-2. `$BEADS_DB` environment variable
-3. `.beads/*.db` in current directory or ancestors
-4. `~/.beads/default.db` as fallback
+2. `$BEADS_DIR` environment variable (points to .beads directory)
+3. `$BEADS_DB` environment variable (deprecated, points to database file)
+4. `.beads/*.db` in current directory or ancestors
 
 **Project-local** (`.beads/`): Project-specific work, git-tracked
 
-**Global fallback** (`~/.beads/`): Cross-project tracking, personal tasks
+**Recommended**: Use `BEADS_DIR` to point to your `.beads` directory, especially when using `--no-db` mode
 
 ---