feat(core): reclaim deleted event aggregates

[LeXwDeX]

Closes #524

Why

Explicit session deletion removed the session aggregate but could strand related DAG event aggregates, and replaying those zombies fails the workflow/session foreign key. SQLite also kept deleted pages allocated because new databases did not enable full auto-vacuum before WAL and legacy conversion had no safe explicit path.

What changed

• Capture and scrub all session-owned DAG aggregates, including terminal workflows, while preserving interruptions and soft-degrading ordinary cleanup failures.
• Add an atomic, guarded startup residue sweep that deletes only aggregates with neither a session nor workflow read model.
• Initialize genuinely new databases with auto_vacuum=FULL before WAL in both drivers; detect legacy NONE databases without converting at startup.
• Add explicit opencode db vacuum --db <existing-file> conversion with target validation and FULL readback verification.
• Wire the sweep through AppLayer and HttpApiApp LayerNode composition.
• Record the approved decision and non-goals in ADR-0001; archived retention, active-history truncation, incremental vacuum, and chore(ops): reclaim pre-guard session data from the local database #531 live cleanup remain excluded.

Evidence

• Focused core tests: 11 passed.
• Focused opencode tests: 11 passed.
• DAG core gate: 60 passed with coverage floors.
• Package and root typechecks passed.
• Root lint passed within the 4850 warning ratchet.
• Full DAG verification and final review: ACCEPT, 0 blocking findings.
• Core full suite after hermetic npm-audit fixture repair: 1228 passed; repair review ACCEPT.
• All database tests use :memory: or disposable temporary files; no live OpenCode database was opened by the delivery tests.

Checklist

• Decision Checkpoint approved and committed as ADR
• Atomic TOCTOU regression pinned
• Startup failure paths soft-degrade while interrupts propagate
• Legacy conversion is explicit and refuses missing/non-file targets
• No migration, HTTP schema, SDK, sync, or event manifest change