fix(mail): slice filesystem mail polling

[BatalloLu]

Summary

• Poll subscribed pseudo-agent outboxes before and after bounded own-inbox slices so human/pseudo outbox delivery cannot be starved behind large historical inbox scans.
• Keep own-inbox scan progress with an iterator cursor and configurable time/entry budgets.
• Add slow-phase telemetry with message body/subject/content/attachment redaction.
• Add focused filesystem-mail tests for pseudo-outbox rechecks between chunks and body-free telemetry.

Context

This targets the filesystem-mail starvation seen on large/external-disk .lingtai mailboxes: a full own-inbox scan can monopolize the mail listener loop long enough that a fresh human pseudo-agent outbox message waits for the next loop. The change keeps the listener responsive to pseudo outboxes while still making incremental progress through the agent's own inbox.

Validation

• git diff --check
• python3 -m py_compile src/lingtai/kernel/services/mail.py
• uv run --with pytest python -m pytest tests/test_filesystem_mail.py -q (25 passed)

Not covered / follow-up

• No live external-disk Wenda validation was performed in this PR.
• No runtime wheel, installed venv, hotpatch, release, or Wenda mailbox/runtime state was changed.