Address gosec security warnings (102 issues)

Security linter warnings: file permissions (0755 should be 0750), G304 file inclusion via variable, G204 subprocess launches. Many are false positives but should be reviewed.


Need comprehensive daemon health check command (bd daemon doctor?)


Audit all cmd/bd tests and group into suites

Analyze all 279 tests in cmd/bd and identify:
1. Which tests can share DB setup (most of them\!)
2. Which tests actually need isolation (export/import, git ops)
3. Optimal grouping into test suites

Create a mapping document showing:
- Current: 279 individual test functions
- Proposed: ~10-15 test suites with subtests
- Expected speedup per suite

Blocks all refactoring work.


bd migrate --update-repo-id triggers auto-import that resurrects deleted issues

**Bug:** Running `bd migrate --update-repo-id` can resurrect previously deleted issues from git history.

## What Happened

User deleted 490 closed issues:
- Deletion committed successfully (06d655a) with JSONL at 48 lines
- Database had 48 issues after deletion
- User ran `bd migrate --update-repo-id` to fix legacy database
- Migration triggered daemon auto-import
- JSONL had been restored to 538 issues (from commit 6cd3a32 - before deletion)
- Auto-import loaded the old JSONL over the cleaned database
- Result: 490 deleted issues resurrected

## Root Cause

The auto-import logic in `cmd/bd/sync.go:130-136`:
```go
if isJSONLNewer(jsonlPath) {
    fmt.Println("→ JSONL is newer than database, importing first...")
    if err := importFromJSONL(ctx, jsonlPath, renameOnImport); err != nil {
```

This checks if JSONL mtime is newer than database and auto-imports. The problem:
1. Git operations (pull, merge, checkout) can restore old JSONL files
2. Restored file has recent mtime (time of git operation)
3. Auto-import sees "newer" JSONL and imports it
4. Old data overwrites current database state

## Timeline

- 19:59: Commit 6cd3a32 restored JSONL to 538 issues from d99222d
- 20:22: Commit 3520321 (bd sync)
- 20:23: Commit 06d655a deleted 490 issues → JSONL now 48 lines
- 20:23: User ran `bd migrate --update-repo-id`
- Migration completed, daemon started
- Daemon saw JSONL (restored earlier to 538) was "newer" than database
- Auto-import resurrected 490 deleted issues

## Impact

- **Critical data loss bug** - user deletions can be undone silently
- Affects any workflow that uses git branches, merges, or checkouts
- Auto-import has no safety checks against importing older data
- Users have no warning that old data will overwrite current state

## Fix Options

1. **Content-based staleness** (not mtime-based)
   - Compare JSONL content hash vs database content hash
   - Only import if content actually changed
   - Prevents re-importing old data with new mtime

2. **Database timestamp check**
   - Store "last export timestamp" in database metadata
   - Only import JSONL if it's newer than last export
   - Prevents importing old JSONL after git operations

3. **User confirmation**
   - Before auto-import, show diff of what will change
   - Require confirmation for large changes (>10% issues affected)
   - Safety valve for destructive imports

4. **Explicit sync mode**
   - Disable auto-import entirely
   - Require explicit `bd sync` or `bd import` commands
   - Trade convenience for safety

## Recommended Solution

Combination of #1 and #2:
- Add `last_export_timestamp` to database metadata
- Check JSONL mtime > last_export_timestamp before importing
- Add content hash check as additional safety
- Show warning if importing would delete >10 issues

This preserves auto-import convenience while preventing data loss.

## Files Involved

- `cmd/bd/sync.go:130-136` - Auto-import logic
- `cmd/bd/daemon_sync.go` - Daemon export/import cycle
- `internal/autoimport/autoimport.go` - Staleness detection

## Reproduction Steps

1. Create and delete some issues, commit to git
2. Checkout an earlier commit (before deletion)
3. Checkout back to current commit
4. JSONL file now has recent mtime but old content
5. Run any bd command that triggers auto-import
6. Deleted issues are resurrected


Baseline quality gate failure: lint

The lint quality gate is failing on the baseline (main branch).

This blocks the executor from claiming work until fixed.

Error: golangci-lint failed: exit status 1

Output:
```
cmd/bd/search.go:39:12: Error return value of `cmd.Help` is not checked (errcheck)
			cmd.Help()
			        ^
cmd/bd/clean.go:118:15: G304: Potential file inclusion via variable (gosec)
	file, err := os.Open(gitignorePath)
	             ^
cmd/bd/doctor/gitignore.go:98:12: G306: Expect WriteFile permissions to be 0600 or less (gosec)
	if err := os.WriteFile(gitignorePath, []byte(GitignoreTemplate), 0644); err != nil {
	          ^
cmd/bd/merge.go:121:16: G204: Subprocess launched with variable (gosec)
			gitRmCmd := exec.Command("git", "rm", "-f", "--quiet", fullPath)
			            ^
cmd/bd/doctor.go:167:20: `cancelled` is a misspelling of `canceled` (misspell)
		fmt.Println("Fix cancelled.")
		                 ^
cmd/bd/flush_manager.go:139:42: `cancelling` is a misspelling of `canceling` (misspell)
		// Send shutdown request FIRST (before cancelling context)
		                                       ^
cmd/bd/flush_manager.go:261:15: `cancelled` is a misspelling of `canceled` (misspell)
			// Context cancelled (shouldn't normally happen)
			           ^
cmd/bd/flush_manager.go:269:55: (*FlushManager).performFlush - result 0 (error) is always nil (unparam)
func (fm *FlushManager) performFlush(fullExport bool) error {
                                                      ^
8 issues:
* errcheck: 1
* gosec: 3
* misspell: 3
* unparam: 1

```