fix(doctor): add .sync.lock and sync_base.jsonl to gitignore (#980)

* fix(doctor): add .sync.lock and sync_base.jsonl to gitignore

Problem:
- .sync.lock and sync_base.jsonl were missing from GitignoreTemplate
- Files introduced in PR #918 (pull-first sync) appeared as untracked

Solution:
- Add patterns to GitignoreTemplate with explanatory comment
- Add patterns to requiredPatterns for bd doctor validation

Impact:
- Existing users get warning via bd doctor, fix via bd doctor --fix
- New repos get correct gitignore on bd init

Fixes: GH#974

* docs(sync): fix sync_base.jsonl tracking status and add tests

Problem:
- SYNC.md incorrectly documented sync_base.jsonl as "git-tracked"
- No tests validated sync state file gitignore patterns

Solution:
- Update Files Reference table: sync_base.jsonl is "not tracked, per-machine"
- Add TestGitignoreTemplate_ContainsSyncStateFiles
- Add TestRequiredPatterns_ContainsSyncStatePatterns

Relates to: GH#974

cmd/bd/doctor/gitignore.go

@@ -46,6 +46,11 @@ beads.left.meta.json
 beads.right.jsonl
 beads.right.meta.json
 
+# Sync state (local-only, per-machine)
+# These files are machine-specific and should not be shared across clones
+.sync.lock
+sync_base.jsonl
+
 # NOTE: Do NOT add negation patterns (e.g., !issues.jsonl) here.
 # They would override fork protection in .git/info/exclude, allowing
 # contributors to accidentally commit upstream issue databases.
@@ -64,6 +69,8 @@ var requiredPatterns = []string{
 	"*.db?*",
 	"redirect",
 	"last-touched",
+	".sync.lock",
+	"sync_base.jsonl",
 }
 
 // CheckGitignore checks if .beads/.gitignore is up to date