refactor(cli): convert session subcommands to effectCmd

[kitlangton]

Summary

• Convert `session list` and `session delete` from `cmd()` + `bootstrap()` to `effectCmd` + `Effect.ensuring(store.dispose(ctx))`.
• `Session.Service` yielded directly (drops `AppRuntime.runPromise` wrappers).
• `try/catch` around `Session.get` becomes `Effect.catchCause(() => fail(...))` (`Session.get` surfaces `NotFoundError` as a defect, not a typed E).
• Pager spawn stays inside `Effect.promise`.
• The parent `SessionCommand` stays on `cmd()` — group dispatcher only.

Behavioral notes

Subagent purity audit: MOSTLY PURE WITH NOTES.

• One strict improvement: on `session delete `, dispose now runs before exit (legacy `process.exit(1)` bypassed bootstrap's finally). Same stderr text + exit code 1.
• No other diffs across either subcommand.

`db.ts` was considered next but skipped — none of its subcommands use bootstrap or any services (just spawns `sqlite3`, runs JSON migration). Converting would add an unnecessary instance load.

Test plan

• `bun run typecheck`
• `bun run test test/cli/` — 137 pass
• Smoke: `session list --max-count 3` prints sessions table
• Smoke: `session delete bogus-id` → `Error: Session not found: bogus-id` (matches legacy)