fix(cli): enable cross-domain referencing for installed knowledges

[MajorLift]

Closes #88.

The defect

Knowledge is copied per domain, so a skill could only ever cite its own domain's files. Four skills cite testing-layers.md, which lives in domains/testing/knowledge/:

Skill	Its domain	Cites from
coding-guidelines (mobile overlay, ×3 sites)	coding	testing
pr-guidelines (mobile overlay)	pr-workflow	testing
pr-readiness-check (mobile overlay)	pr-workflow	testing
perps-review-pr	perps	testing

Six citation sites, none of which could resolve for any consumer on any operator. The skill installed fine and the reference dangled — no error, the agent simply could not open what the body pointed at.

The fix

The installer scans a skill for knowledge/<file>.md citations its own domain does not satisfy, and copies the file in from whichever domain owns it.

Existing citations start working unchanged — no new syntax to learn, and nothing is declared in frontmatter, so the context an agent must load stays bounded. A cross-domain reference remains a body link the agent chooses to follow.

Overlays are scanned too. The repo overlay is merged into the emitted body, so a citation there is just as load-bearing — and three of the four cases cite from repos/metamask-mobile.md rather than skill.md. Scanning only the base fixed one of four, which is how I found it.

Ambiguity fails loudly. Resolution is by filename, which is unambiguous today — ten knowledge files, no name shared across domains. If that stops being true the reference is genuinely ambiguous, so the install exits non-zero and names the candidates rather than silently picking one.

Evidence

Real corpus, installing coding,perps,pr-workflow,testing for metamask-mobile:

before   9/15 knowledge citations resolve
after   15/15

Three tests, all failing against main's installer and passing here:

main    ℹ pass 12   ℹ fail 2
branch  ℹ pass 14   ℹ fail 0

• a skill receives knowledge it cites from another domain, on all three operator outputs
• the owning domain still gets its own knowledge
• an ambiguous filename fails rather than picking one

Notes

• No CHANGELOG.md entry — this is install behavior, so say the word if you want one and which release it lands under.
• Complements test(ci): Enforce that knowledge/ installations resolve #87, which guards that knowledge/ citations resolve after install. That guard is what makes this class visible; this closes the remaining cases it surfaced.