Harden multi-agent skill installation

[forhappy]

Summary

• add automatic project/user scope resolution, multi-agent detection, repeatable platform selection, shared/native skill destinations, dry-run, strict, require-all, and JSON/text reports
• harden install and uninstall with ownership manifests, digests, exact managed sections/hooks/plugins, scope locks, staged activation, rollback, symlink rejection, purge containment, and legacy Codex migration safety
• guide Codex, Claude Code, Gemini, OpenCode, Copilot, Kiro, Cline, Cursor, Kilo, Devin/Windsurf, Antigravity, Amp, and other agent-skills consumers with Graphify-style graph-first codebase instructions
• document the workflow and add cross-platform lifecycle and failure-path coverage

Verification

• cargo test -p compass-cli
• cargo test -p compass-cli --test install_cli (21 tests)
• cargo clippy -p compass-cli --all-targets --no-deps -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• graphify update .

An independent final code review found no remaining release-blocking correctness or filesystem-safety gaps.