[fnzko]

What does this PR do?

Depends on #354 (until it and #348 merge, only the last 2 commits are new here).

A plugin can pass cargo agents plugin validate and still not work the same on every agent: Antigravity rejects a . in its name, agents that cache by version may not refresh a package with no version, and an agent-format hook fires on that agent alone. A skill file that links outside its package is left out of the install, and only sync says so.

Now validate warns about each of these, and about every skill entry the install would skip (broken links and loops too), using the walk the copy uses. Warnings never fail the command; each is its own report line.

Verified with a unit test per warning and the existing plugin validate snapshot.

Take into account:

• fix: issues found by end-to-end testing #366 and the crate validation PR change the same function; whichever merges second rebases.
• Checks that need the agent itself are left to plugin try.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code

[fnzko]

Folded into #373.