tests: verify installed skill content (not just existence) in verbose-docs tryscript

## Summary

The `verbose-docs.tryscript.md` skill tests (V6–V12, added in v0.7.2) give great
end-to-end coverage of the cross-agent `--install-skill --surfaces` flag, but they verify
each surface only with `test -f` (file existence). They don't read the installed files to
confirm the **content** landed correctly (right frontmatter, the `DO NOT EDIT` format
stamp, the marker-bounded `AGENTS.md` block).

A skill install that wrote a truncated/garbled file, or the right bytes to the wrong path,
would still pass the existence checks. Adding a content assertion closes that gap and makes
the golden test exercise what actually matters: *the correct content at the correct path*.

## Proposed additions (two scenarios)

```console
## V13: Installed Claude skill has the right content at the right path
$ mkdir v13 && cd v13 && flowmark --install-skill --surfaces=claude >/dev/null && grep -Fx "name: flowmark" .claude/skills/flowmark/SKILL.md && grep -F "DO NOT EDIT" .claude/skills/flowmark/SKILL.md
name: flowmark

## V14: Installed AGENTS.md block has marker-bounded content
$ mkdir v14 && cd v14 && flowmark --install-skill --surfaces=agents-md >/dev/null && grep -Fx "" AGENTS.md && grep -Fx "" AGENTS.md

```

## Context

These were added to the Rust port (flowmark-rs) while porting v0.7.2's cross-agent skill
install. The port keeps its `verbose-docs.tryscript.md` in sync with this file; filing here
so the Python reference picks up the same content-level coverage and the two stay aligned.

(Reported from the flowmark-rs v0.7.0 → v0.7.2 sync.)