feat(cleanup): protect pinned issues from cleanup/compact (bd-b2k)

Add Pinned field to Issue struct and database schema to protect
issues from accidental deletion via cleanup or compaction.

Changes:
- Add Pinned bool field to types.Issue
- Create migration 023_pinned_column.go for database schema
- Filter out pinned issues in cleanup command before deletion
- Add pinned check to GetTier1Candidates and GetTier2Candidates
- Add pinned check to CheckEligibility for compaction
- Update all SQL queries and scan functions to include pinned field

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/schema.go

@@ -32,6 +32,8 @@ CREATE TABLE IF NOT EXISTS issues (
     ephemeral INTEGER DEFAULT 0,
     -- NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
     -- These relationships are now stored in the dependencies table
+    -- Protection fields (bd-b2k)
+    pinned INTEGER DEFAULT 0,
     CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 );