fix(init): respect BEADS_DIR environment variable (#1273)

* fix(sync): read sync.mode from yaml first, then database

bd config set sync.mode writes to config.yaml (because sync.* is a
yaml-only prefix), but GetSyncMode() only read from the database.

This caused dolt-native mode to be ignored - JSONL export still
happened because the database had no sync.mode value.

Now GetSyncMode() checks config.yaml first (via config.GetSyncMode()),
falling back to database for backward compatibility.

Fixes: oss-5ca279

* fix(init): respect BEADS_DIR environment variable

Problem:
- `bd init` ignored BEADS_DIR when checking for existing data
- `bd init` created database at CWD/.beads instead of BEADS_DIR
- Contributor wizard used ~/.beads-planning as default, ignoring BEADS_DIR

Solution:
- Add BEADS_DIR check in checkExistingBeadsData() (matches FindBeadsDir pattern)
- Compute beadsDirForInit early, before initDBPath determination
- Use BEADS_DIR as default in contributor wizard when set
- Preserve precedence: --db > BEADS_DB > BEADS_DIR > default

Impact:
- Users with BEADS_DIR set now get consistent behavior across all bd commands
- ACF-style fork tracking (external .beads directory) now works correctly

Fixes: steveyegge/beads#???

* fix(doctor): respect BEADS_DIR environment variable

Also updates documentation to reflect BEADS_DIR support in init and doctor.

Changes:
- doctor.go: Check BEADS_DIR before falling back to CWD
- doctor_test.go: Add tests for BEADS_DIR path resolution
- WORKTREES.md: Document simplified BEADS_DIR+init workflow
- CONTRIBUTOR_NAMESPACE_ISOLATION.md: Note init/doctor BEADS_DIR support

* test(init): add BEADS_DB > BEADS_DIR precedence test

Verifies that BEADS_DB env var takes precedence over BEADS_DIR
when both are set, ensuring the documented precedence order:
--db > BEADS_DB > BEADS_DIR > default

* chore: fill in GH#1277 placeholder in sync_mode comment

cmd/bd/doctor.go

@@ -144,9 +144,15 @@ Examples:
 		// Use global jsonOutput set by PersistentPreRun
 
 		// Determine path to check
-		checkPath := "."
+		// Precedence: explicit arg > BEADS_DIR (parent) > CWD
+		var checkPath string
 		if len(args) > 0 {
 			checkPath = args[0]
+		} else if beadsDir := os.Getenv("BEADS_DIR"); beadsDir != "" {
+			// BEADS_DIR points to .beads directory, doctor needs parent
+			checkPath = filepath.Dir(beadsDir)
+		} else {
+			checkPath = "."
 		}
 
 		// Convert to absolute path