Stabilize hook CopyFiles fingerprints

[julianknutsen]

RCA

Desired-state hashing was recording CopyFiles before runtime provider overlays had materialized and canonicalized hook files in the workdir. On the next reconciliation tick, those Gas City-managed hook artifacts had different content hashes, so sessions drained for config-drift even though operator intent had not changed.

.gc/scripts remains in CopyFiles as requested, because script changes are intended to trigger churn.

Fix

• Materialize effective pack/agent provider overlays before desired-state fingerprinting.
• Keep copied hook files scoped to the resolved provider and requested install-hook slots.
• Preserve Claude city settings handling and .gc/scripts drift behavior.
• Add regression coverage proving runtime staging no longer changes the core fingerprint.

Validation

• go test ./cmd/gc -run 'TestBuildDesiredState_MaterializesHookOverlaysBeforeFingerprinting|TestBuildDesiredState_InstallsGeminiHooksBeforeFingerprinting|TestStageHookFiles' -count=1
• go test ./internal/runtime ./internal/overlay -count=1
• go test ./cmd/gc -count=1

Bead: mc-jkh5n4t