fix(gateway): clear queued reload skills notes on new/resume/branch

[molvikar]

Summary

This fixes a stale gateway state leak where the one-shot note queued by /reload-skills
could survive a session boundary and get prepended to the next message in a different
conversation branch.

The queued note lives in self._pending_skills_reload_notes[session_key] and is normally
consumed on the next user turn. Before this change, session-boundary flows like /new,
/resume, and /branch cleared approval/update state but did not clear the pending
reload-skills note.

What changed

• Clear _pending_skills_reload_notes[session_key] inside
  GatewayRunner._clear_session_boundary_security_state()
• Extend gateway boundary regression tests to verify that /resume, /branch,
  and the shared cleanup helper remove the queued note only for the target session

Why this matters

Without this fix, a user could run /reload-skills, then switch/reset/branch the session,
and see the old reload note injected into the first message of the new session context.
That is a small but real session-boundary state leak.

Tests

Passed:

• tests/gateway/test_session_boundary_security_state.py

• tests/gateway/test_reload_skills_command.py

• Result: 9 passed

[teknium1]

Salvaged via #19431 onto current main — your commit's authorship was preserved. Thanks @molvikar!