refactor: decompose CLI, AccountManager, and orchestration into modules

Hotspot decomposition per .sisyphus/plans/quality-refactor.md:

- src/cli.ts: thin dispatcher routing to extracted command handlers in
  src/cli/commands/{auth,config}.ts and src/cli/formatting.ts.
- src/index.ts: orchestration path flattened via helpers in
  src/request-orchestration-helpers.ts and the existing plugin/refresh
  helper modules; no public hook signatures changed.
- src/accounts.ts: persistence and reconciliation responsibilities split
  out into src/accounts/{persistence,matching}.ts with companion tests.
- src/commands/router.ts: reduced to routing + thin command coordination
  with command bodies extracted to src/commands/handlers/{betas,config,
  files}.ts and src/commands/oauth-flow.ts.

No public CLI command signatures or documented plugin behavior changed.
Test count unchanged on touched paths; new modules have direct tests.