fix: forge install freshness warning + qualifier-aware prune

[N4M3Z]

Closes #73

Problem

forge install deploys whatever the source working copy holds, so an install from a working copy parented behind trunk silently re-deploys already-merged-away content. And prune leaves a stale deployed base file alive when its source counterpart exists only as a qualifier variant.

Fix

1. Freshness check: before deploying, compare the source git working copy against its trunk ref (origin/main, no network fetch). If behind, refuse with a prominent warning unless --allow-stale. A git-read failure is advisory, never fatal to install.
2. Qualifier-aware prune: a deployed base file whose source counterpart exists only under a qualifier directory is pruned; a correctly-resolved qualifier deployment is kept.

Test plan

• fmt / clippy -D warnings / test
• freshness: stale source refuses without --allow-stale, proceeds with it, non-git skips silently
• prune: base pruned when only an inactive model variant exists; kept when a real base or active variant resolves
• CI matrix

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