Multi-account support and correct live-store selection

[joshthomp]

Summary

RemCTL currently reads from a single Reminders store chosen by file size.
On setups with more than one account (iCloud + Exchange/CalDAV + on-device),
the largest file is often a bloated Exchange store rather than the live
iCloud one, so RemCTL silently operates on the wrong account.

This PR fixes store selection and, on top of that, adds opt-in
multi-account support: read from every connected account, see which account
each item belongs to, and create/modify/delete in any of them. The default
behavior is unchanged and single-account output is byte-for-byte identical.

Motivation

find_main_db_path() selected the store with the largest file on disk.
Exchange stores accumulate size independent of how many live reminders they
hold, so the heuristic can pick a stale/secondary account. The result is
confusing: today, lists, etc. show an account the user didn't expect.

What changed

Store selection

• Choose the active store by reminder count, then file-group recency.
• REMCTL_DB env var / dbPath config key to pin a specific file;
  REMCTL_STORE_DIR / storeDir to point at a Stores directory.

Reading across accounts (opt-in)

• --all-accounts includes every connected account; --account NAME
  restricts to one. A personal default can be set with
  REMCTL_ACCOUNT_SCOPE or the accountScope config key (all or a name).
• Source labelling: bold per-account headers (human), account /
  accountType fields (JSON), an Account column (table).
• IDs (Z_PK) are per-account; when the scope spans accounts, an ID or
  list name found in more than one produces a disambiguation error that
  lists candidates and asks for --account.
• New accounts command (lists connected accounts) and config command
  (accountScope / storeDir / dbPath).

Writing to a specific account

• The EventKit bridge gains a list_calendars action and targets lists by
  stable calendar identifier, so a (list, account) pair resolves to the
  right calendar instead of an arbitrarily-chosen same-named list.
• --account on add / done / edit / delete / flag, etc.
• ⚠️ The bridge must be recompiled after pulling this change:
  ./install.sh (runs swiftc -O).

Correctness fix

• stats counted overdue against the current time, so reminders due earlier
  today were reported as overdue even though overdue itself lists none.
  It now counts from the start of the day, matching overdue and today.

Deliberate scope limit

• export / import operate on a single account (reminder IDs collide
  across accounts). They accept --account but refuse an all-accounts
  scope with a clear error rather than writing a silently partial backup.

Backward compatibility

Single-account behavior is the default. With no flags/config, output is
byte-for-byte identical to the previous release — verified by diffing every
read command × format (plain/table/json) against main, for both the
default scope and an explicit --account. The only intended output change
is the stats overdue fix above.

Testing

• python3 -m pytest tests/test_cli.py -> 273 passing, plus coverage for
  account discovery, scope precedence, cross-account addressing/ambiguity,
  and single-account back-compat.
• One pre-existing failure,
  test_info_json_keeps_due_date_separate_from_display_alarm_date, is
  not introduced here — it fails identically on unmodified main
  because it hard-codes a UTC+2 expected value and only passes in that
  timezone. Happy to fix it (force a fixed TZ) in a separate change if you'd
  like.

Notes for review

• One PR intentionally bundles the Python CLI and the Swift bridge because
  account-targeted writes need both.
• No third-party dependencies added.