fix: support configurable IMAP mailboxes

[pilipilisbot]

Summary

• make gab read-imap-once --mailbox default to GITHUB_AGENT_BRIDGE_MAILBOX, matching the systemd reader wrapper
• centralize IMAP mailbox quoting so direct CLI reads can select folders with spaces, such as [Gmail]/All Mail
• update operator docs and env example for shared-account mailbox/folder configuration

Tests

• .venv/bin/python -m pytest tests/test_reader_run.py -q
• .venv/bin/python -m pytest -q
• git diff --check

No screenshots: backend/CLI/docs-only change.

Closes #47

[pilipilisbot]

Post-merge cleanup complete for PR #48.

• Merge commit: 559ac6124873c663bd592b55b27fab5278bae7c2
• Post-merge GitHub Actions passed: tests run 26408974765, release run 26408974747
• Published release: v0.18.6
• No dedicated PR worktree existed under the workspace, so there was no worktree directory to remove.
• Removed the stale local support-imap-mailboxes branch after GitHub deleted the remote head branch.
• Left the canonical checkout in place; it is clean and currently on unrelated branch fix-url-priority.