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

docs/WORKTREES.md

@@ -448,6 +448,20 @@ For users who want complete separation between code history and issue tracking,
 
 ### Setup
 
+**Option A: Initialize with BEADS_DIR (simplest)**
+```bash
+# 1. Create the directory structure
+mkdir -p ~/my-project-beads/.beads
+
+# 2. Set BEADS_DIR and initialize from anywhere
+export BEADS_DIR=~/my-project-beads/.beads
+bd init --prefix myproj    # Creates database at $BEADS_DIR
+
+# 3. Initialize git in the beads repo (optional, for sync)
+cd ~/my-project-beads && git init
+```
+
+**Option B: Traditional approach**
 ```bash
 # 1. Create a dedicated beads repository (one-time)
 mkdir ~/my-project-beads