Code domain: implement directory-level insight dimension

Background

The @gabriel/code domain currently declares three dimensions: symbol, file, and directory. The first two have engine-level support. directory was added to the domain capabilities to reflect the semantic reality that directory structure carries architectural intent — src/, tests/, api/routes/ are not neutral containers, they are part of the design.

We opted out of .musekeep precisely because directory layout has meaning. A file moving between directories is a meaningful change even when its content is identical. This dimension should be a first-class citizen.

What directory dimension insight means

Unlike symbol and file, a directory has no byte content — its content is its membership and its position in the tree. So directory-level insight is:

• Membership diff: what files moved in or out of a directory between two refs
• Structural diff: did the directory itself move, rename, or disappear
• Churn: which directories change most often (hotspot at the architectural level)
• Coupling: which directories always change together (entanglement at module level)
• Blast radius: if this directory's structure changes, what else is affected

What needs to be implemented in muse

• InsightProvider for the code domain should emit a directory dimension in its insight output
• muse divergence should include directory-level divergence scores between branches
• muse diff should surface directory-level structural changes (moves, renames, deletions) as a distinct dimension, not just a side-effect of file diffs
• muse code hotspots and muse code entangle should be able to operate at directory granularity, not just symbol/file
• muse code impact should optionally roll up blast radius to the directory level

Current state

• Domain capabilities: dimensions: [symbol, file, directory] ✅
• Engine insight output for directory: not implemented
• GUI stat strip on domain detail page: shows 3 dimensions ✅

Notes

This is a parity ticket — the dimension is declared, the engine needs to catch up. No MuseHub changes required.