[Dkm0315]

CI and route fixes for four issues.

What the user can now do

• With "Let agents use devices" on, a Full access Claude chat on a router runs over the router's Messages API again. Before, the device tool server kept every such turn on the Codex client. The Messages route drops the device server and its note, as it already did for the browser, canvas, office and delegation servers. The Claude Code route keeps the device tools.
• CI on Linux no longer fails when one apt mirror can't find xvfb, xauth or desktop-file-utils, or stalls. scripts/ci-apt-install.sh points the Ubuntu sources at kernel.org, then Azure, then Ubuntu's archive, and makes two passes over the list with a time limit on each attempt. The Linux build job and the Ubuntu 24.04 container job both use it.
• Two flaky tests are now reliable:
  • acp-route-160: follow-ups wait for the previous turn to settle.
  • day-dividers: the test counts timestamp parses instead of timing them.

Spec cases

• MUS-SPEC-03-866A (new): device tools don't move a Claude turn off the Messages API.
• MUS-SPEC-03-866B (new): Claude Code on a router keeps the device tools. This is a guard; it also passes before the change.
• MUS-SPEC-20-770A (new): day dividers don't re-parse timestamps.
• MUS-SPEC-22-876A and 22-876B (new): the apt step falls back across mirrors, and both Linux jobs use it.
• MUS-SPEC-03-160D and 160E: the Environment field now records the 150 ms end delay and the settle-before-send rule.

Red, at fe91b85 (node --experimental-transform-types --import ./tests/pin-device-platform.mjs --test --test-concurrency=1 tests/<file>)

• claude-messages-lane: exit 1, 7 passed and 1 failed. The 866A turn went to the Codex client.
• acp-route-160: exit 1, 8 passed and 2 failed, both "Chat is already running; stop it first". The fake agent now answers 150 ms after its last words, which makes the CI race deterministic.
• ci-apt-install: exit 1, 0 passed and 2 failed (there was no script yet).
• day-dividers: this was a flake, so there was no red on main. As a mutation check, I disabled the parse cache in dayDividers.ts; the test then failed with 3 passed and 1 failed.

Green, at a4f0209 (same command)

• claude-messages-lane: 8 passed, 0 failed. It also passes with MUSTER_DEVICE_MCP_LAUNCHER set in the environment, because the harness now sets or clears every launcher.
• router-claude-code-730: 5 passed, 0 failed.
• acp-route-160: 10 passed, 0 failed.
• day-dividers: 4 passed, 0 failed.
• ci-apt-install: 2 passed, 0 failed.
• delegation-44 and office-skills-772: 12 passed each.
• resource-errors and device-noun: 4 passed each.
• check-spec: 812 cases OK.
• check-test-coverage: no orphan files.
• npm run typecheck: exit 0.

Not verified

• The apt script hasn't run against real mirrors locally. It ran under bash 3.2 on macOS with a stand-in apt-get. This PR's Linux jobs are the end-to-end check.
• computer-mcp needs a built dist, so it wasn't run locally. CI builds and runs it.
• Locally, acp-route-160 failed 3 tests on a clean checkout because the safe-write helper wasn't built. npm test builds it in pretest, so this isn't a bug.

Fixes #866
Fixes #877
Fixes #770
Fixes #876