refactor: remove legacy kb compatibility

Drops the .kb/kb.db migration path, legacy backup filename handling, and
backward-compat test suites. Renames internal kbDir identifiers to
fusionDir and hasKbProject/isValidKbProject to their fusion equivalents.

- Remove needsCentralMigration, autoMigrateToCentral, and the
  "needs-migration" FirstRunState; checkAndMigrate and KB_SKIP_MIGRATION
  env var are gone
- Remove LEGACY_BACKUP_DIR and canonicalizeBackupDir; listBackups no
  longer matches kb-* filenames
- Delete backward-compat.test.ts and store-backward-compat.test.ts;
  update remaining tests to new 3-state first-run model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/skill/fusion/references/fusion-capabilities.md

@@ -85,9 +85,8 @@ Triage → Todo → In Progress → In Review → Done → Archived
 ## Task Storage Structure
 
 ```
-.kb/
-├── kb.db                    # SQLite database (WAL mode)
-├── config.json              # Board config
+.fusion/
+├── fusion.db                # SQLite database (WAL mode)
 └── tasks/
     └── KB-001/
         ├── PROMPT.md        # Task specification