fix(matrix): filter phantom interrupt / redirect notices at intake

[Wintle]

Summary

The matrix adapter's intake gate (_on_room_message) now drops messages
whose body matches an orchestrator-emitted phantom notice shape BEFORE any
sender / room / duplicate filtering.

Phantom shapes filtered (full-prefix match, real user messages with these
tokens mid-sentence still pass):

• [This response was interrupted by a user correction.]
• ↪ Redirected current run
• ⚡ Interrupting current task.
• ⚡ Stopped.
• No active task to stop.
• ♻️ Recovered reply
• 💾 Self-improvement review:
• 💭 Reasoning:

Why

Hermes orchestrator notices (interrupt / redirect / self-improvement
review / recovered-reply) get delivered to the gateway as ordinary
inbound messages. Without a defensive filter the matrix adapter dispatches
them, the LLM treats them as a user turn, and the gateway emits another
phantom — a self-perpetuating loop until the homeserver rate-limits the
connection. Filtering at intake is the cheapest point to break the loop.

Why this is a separate PR from #80206 (sender/reply metadata)

• fix(matrix): propagate sender MXID + reply context to MessageEvent #80206 fixes an affordance gap — MessageEvent now carries user_id /
  user_name / reply_to_* so the LLM CAN read sender info.
• This PR is a defensive intake filter — orchestrator noise NEVER reaches
  the LLM.

Different surface area, different risk profile, different rollback path.
AGENTS.md contribution rubric asks for focused PRs — these two fixes are
independently mergeable.

Verification

RED on origin/main (stashed the source fix, kept the tests):

$ git stash push -- plugins/platforms/matrix/adapter.py
$ ./venv/bin/python -m pytest tests/gateway/test_matrix_phantom_intake_filter.py
9 failed, 2 passed in 0.98s

GREEN after fix (popped the stash):

$ git stash pop
$ ./venv/bin/python -m pytest tests/gateway/test_matrix_phantom_intake_filter.py
11 passed in 0.92s

Sibling matrix tests (test_matrix*.py plus the new file):
all green.

Files changed

• plugins/platforms/matrix/adapter.py:346 — new module-level
  PHANTOM_INTAKE_PREFIXES tuple + _is_phantom_intake_message() helper
  (full-prefix matching, rejects empty body).
• plugins/platforms/matrix/adapter.py:3127 — _on_room_message now
  inspects event.content.body and returns early if the body matches a
  phantom prefix. Placed BEFORE the existing self-sender / allowed-room /
  duplicate-event gates so phantoms are dropped before any state writes.
• tests/gateway/test_matrix_phantom_intake_filter.py (new) — 11 tests:
  parametrized coverage of every prefix shape, plus a "real message that
  happens to mention the tokens mid-sentence still passes" guard test.

Bot / duplicate check

• Open issues referencing this: none found
• Open PRs referencing this: none found
• No existing branch search needed (no PR competitor)