Report the install directory in skills get output (#573)

Fixes #552.

After installing, the per-agent summary named the agent but not where the
skills actually went. It now names the directory:

Before:

    Installed 2 skill(s) for generic.
    Installed 2 skill(s) for claude.

After:

    Installed 2 skill(s) for generic at .agents/skills.
    Installed 2 skill(s) for claude at .claude/skills.

This follows the display you added to `skills list` in #554 (d1405de), whose
header is `claude (installed at .claude/skills):` — same project-relative
directory, same `at` preposition. The value here is `agent.skillsRelativePath`
rather than `p.split(p.relative(adapter.skillsDirectory, from: rootPath)).join('/')`;
I checked all six adapters on multiple roots and the two produce identical
strings, so this avoids pulling `package:path` into `get_skills.dart` while
staying consistent with what `list` prints.

Not included: a location on each per-skill line. The directory is constant per
agent, so repeating it on every skill is noise, and `InstalledSkillInfo`
carries no path — threading one through `_installSkills` →
`_PackageInstallResult` → `SkillInstallResult` would multiply the diff for that
noise. Happy to do it if you disagree.

This touches `get_skills.dart` and the `## 1.0.0-wip` CHANGELOG, the same two
files as your open #568. I merged the two locally and they apply cleanly, so
there's nothing to coordinate.

Test covers two agents with different directories in a single `get` run.