Correct the Dev Box Stop qualification contract

[coneilen]

Summary

The beta9 Dev Box run failed section 6A because the runbook required Stop loop
to end the loop's zmx session. Stop is the reversible verb. For a reachable
session, GraphStore.requestStop types MessageBus.stopRequest into it — a
request that tells the agent to stay in the session — and kills the session only
when it cannot be reached. This PR replaces that expectation with the contract
the product actually implements, and makes the daemon's memory-log claim the
thing that is checked.

Changes

• investigation/windows-preview-devbox-qualification-plan.md: add a Stop
  contract under D7.
  • Stop passes when the loop reaches stopped, no other loop changes state,
    owned sentinel sessions (one unprefixed and one graphcode- decoy) keep
    their PIDs and receive no input, and the shell and daemon stay responsive.
  • The loop's memory-log entry must match what is observed: "asked to stop
    looping" means the session survives and shows the request; "could not be
    reached, so it was killed" means the session is gone within 15 s.
  • A log that claims a kill while the session survives is a
    TerminalSessionOwnership failure. That is the beta9 result, caused by the
    packaged daemon not locating zmx, which is a separate product fix.
• investigation/windows-preview-release-plan.md: note that the beta9
  prompt's "Stop must end the session" step is superseded by this contract.

The immutable beta9 handoff copy of the plan is unchanged; the next handoff
should ship this version. The plan file keeps its existing CRLF line endings, so
git diff --check reports CR on its added lines by convention; the release
plan is clean.

Test plan

Documentation only; no product code or test changed. Evidence is a check that
the contract is absent on the base and present on this branch, plus a link check.

RED: git show origin/main:investigation/windows-preview-devbox-qualification-plan.md, Select-String for the Stop contract heading and the asked-to-stop-looping path -> 0 matching lines
GREEN: same Select-String against HEAD -> 2 matching lines
REGRESSION: Select-String for the stop-contract anchor heading plus git diff --check on investigation/windows-preview-release-plan.md -> 1 heading, exit 0

The underlying behaviour was observed on the beta9 Dev Box:

• shipped layout: the memory log said the session was killed, yet it survived
  with the same PID;
• with zmx reachable (diagnostic): the log said the session was asked to stop
  looping, and it survived as designed.

make test and make check were not run; nothing they cover changed.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test) — not run: documentation-only change, macOS-only target
• Code follows the existing style (make check) — not run: no Swift changed
• I added the test/contract before the implementation and observed the intended RED failure — the RED line above is a base-revision check, not a test written first