fix: two defects that broke famstack on a clean install

[arthware-dev]

Summary

Two unrelated defects found while bringing the memory and agent stacklets up on the dev rig. Neither is Paperless work; they landed on this branch because they blocked getting there. Each is a standalone commit.

stack memory person could not run on a normal install. It died with No module named 'frontmatter' on any host that had not run pip install. The vault loaders imported the third-party python-frontmatter package, which ships only in the test extra. stack memory correspondents had the same defect.

The reason it shipped is worth recording: the test environment installs a dependency the production host does not, so the suite stayed green while the command was broken for every user. tests/stacklets/test_memory_host_stdlib.py closes that hole by blocking the module in sys.modules, making a machine that never ran pip install the thing under test. All four of its tests fail against the old code.

Both loaders now use lib/stack/frontmatter.py, the stdlib-only parser already built for exactly this, keeping the CLI's documented zero-pip-deps promise. List fields go through _fm_list so a single value written where a list belongs can no longer iterate into one entry per character.

That surfaced a third thing: the shipped correspondents seed teaches inline [a, b] list syntax, which the vault format spec excludes and the parser does not read. Seed and test fixture now use block lists. Whether the parser should instead accept flow syntax is a real design question and is deliberately not decided here.

A stacklet whose containers are all optional could never finish setup. When COMPOSE_PROFILES excludes every service, docker compose exits 1 with no service selected. That is an empty selection, not a failure to start, but the CLI read any non-zero as "Failed to start services" and withheld the setup marker. The ai stacklet is the case that surfaced it: STACK_AI_NO_VOICE=1 clears the profile and its only container sits behind voice, so the documented local-dev opt-out could never complete, which in turn blocked everything that requires ai.

The exit code and stderr text pinned in those tests came from a real docker compose up run rather than from reading source, so they assert compose's actual contract.