Remove the lager-mcp console script and clean up the links it left on boxes

[danielrmerskine]

Closes #242.

What was wrong

cli/setup.py declared lager-mcp=lager.mcp.server:main. lager lives under box/ and the
wheel's top level is cli and nothing else, so pip created the script and it died on first use.
Demonstrated against a wheel built from current main:

$ .../bin/lager-mcp
    from lager.mcp.server import main
ModuleNotFoundError: No module named 'lager'

pip does not validate entry-point targets at install time, which is why this shipped in every
release carrying the script and never failed a build. Installing the extra does not help either:
lager-cli[mcp] supplies the PyPI mcp SDK, not the lager package.

Why removal rather than a wrapper

The issue offered three options; the deciding fact is that the console script was never how this
server runs. The box starts it in-container as python3 -m lager.mcp
(box/lager/docker/start-services.sh:59) and serves it on port 8100, which is what
docs/source/reference/mcp/overview.mdx documents and what clients connect to. Shipping
box/lager/mcp/ into the wheel was the other option and is worse than it looks: main() needs
uvicorn and starlette, neither declared anywhere, and the control tools pull in the entire
Net stack.

So there is no working behavior to preserve — only a script that has never done anything but
raise.

The part that affects deployed boxes

lager install / lager update symlinked the script into ~/.local/bin
(cli/commands/utility/_host_cli.py). Boxes deployed from any ref carrying the entry point have
a ~/.local/bin/lager-mcp pointing at a script that cannot run, and simply not creating it going
forward would leave those boxes broken indefinitely. The install command now removes the link,
so they self-heal on the next deploy. Still best-effort (|| true) — a missing link must not fail
an install.

Regression guard

#242 asks that the smoke be added once resolved so it cannot regress. Since the resolution is
removal, the guard asserts absence: the wheel smoke fails if bin/lager-mcp reappears. That
is the honest form of the check here — re-adding a console script whose target the wheel does not
ship would otherwise sail through again.

Verified in both directions by running the guard's condition against two real wheels: it fires on
a wheel built from current main (script present) and passes on this branch's wheel.

Verified

• Wheel built from this branch declares exactly one console script: ['lager']. bin/lager-mcp
  is absent; lager --version and --help work.
• The same build from main produces bin/lager-mcp and the ModuleNotFoundError above — the
  before/after is a real observation, not an inference.
• cli/tests/test_host_cli.py: 41 passed. The old
  test_lager_mcp_symlink_is_guarded_and_best_effort is rewritten to assert the removal line
  (rm -f, no ln -sfn, still || true) rather than the guarded symlink.
• unit (cli): 1224 passed, 2 xfailed — unchanged, so test/COVERAGE.md needs no edit (a test
  was rewritten, not added).
• packaging.yml parses as YAML and every embedded run block passes bash -n.

Left in place

The mcp extra in cli/setup.py stays — .github/workflows/unit-tests.yml installs and
exercises it, and it is independently useful. docs/source/release-notes/v0.3.27.mdx still
mentions running lager-mcp; historical release notes are a record of what was announced at the
time, and the CHANGELOG entry here supersedes it.

Note for whoever merges second

This adds a ## [Unreleased] CHANGELOG section, as do #249, #251 and #253. Whichever lands second
wants a trivial merge into one section. No other overlap — this touches setup.py,
_host_cli.py, its test, and packaging.yml, none of which the others change.