refactor(cli): convert export command to effectCmd

[kitlangton]

Summary

• Convert `cli/cmd/export.ts` from `cmd()` + `bootstrap()` to `effectCmd`.
• `Session.Service` yielded directly (drops three `AppRuntime.runPromise` wrappers).
• Body extracted to `run()` so the legacy `bootstrap()` finally-disposal matches via `Effect.ensuring(store.dispose(ctx))`.
• `try/catch` around session lookup → `Effect.catchCause(() => fail(...))` because `Session.get` surfaces `NotFoundError` as a defect (not a typed E).

Behavioral notes

• Subagent purity audit: PURE.
• One strict improvement: disposal now also runs on the `Session not found` error path (legacy `process.exit(1)` synchronously killed the process before bootstrap's finally could dispose). All other paths and error-formatting are identical.

Test plan

• `bun run typecheck`
• `bun run test test/cli/` — 137 pass
• Smoke `export --help` — yargs help intact
• Smoke `export bogus-session-id` — prints `Error: Session not found: bogus-session-id` (matches legacy)

Independent of #25465 / #25467.