integration: Regenerate conflicted derived files on merge

[ed-irl]

Adds a git-native regenerate merge driver that re-runs the
appropriate generator for derived files when they conflict, rather
than picking a side. Picking a side is unsafe here: many of these
files are only PARTIALLY stochastic. Two branches may have added or
changed real structure (a new flag, test case, mock method) on top of
random ID drift, and a blind take-one silently drops the other side's
structural change. Regenerating against the merged source produces
the correct merged output.

Path mappings, in .githooks/merge-regenerate:

• internal/forge/shamhub/testdata/** -> go test -run TestIntegration
  ./internal/forge/shamhub/ -update
• testdata/help/*.txt -> go test -run TestHelp . -update
• doc/includes/cli-*.md -> mise run generate
• mocks*.go / mock_*.go / mocks.go -> mise run generate

Generators that emit many files in one run are deduped per merge via
a tmp marker keyed by the parent PID, so the shamhub test runs at
most once even if all 47 fixture files in its output set conflict.

mise run setup registers the driver locally; .gitattributes
declares the path mappings. No post-merge hook is needed: the
driver handles conflicts directly during the merge.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • docs: Add shared protocol guide for script integrations #1249
    • scriptrun: Generic runner + shared protocol surface #1188
      • integration: Add state model for integration branch #1165
        • git: Add Merge, IsClean, and CheckoutNewBranch worktree helpers #1172
          • integration: Add handler package for integration branch operations #1173
            • integration: Wire gs integration command tree #1174
              • integration: Auto-rebuild integration branch after restack #1175
                • integration: Regenerate conflicted derived files on merge #1176 ◀
                  • integration: Add AI-driven conflict resolution #1251
                  • integration: Move regeneration out of merge driver #1252
                  • integration: accept-incoming fallback after auto-resolve #1253
                  • integration: submit tip downstacks alongside the integration branch #1254
                  • integration: surface integration branch and tips in gs ll #1255
                  • integration: short-circuit gs branch checkout for the integration #1256
                  • integration: gs ss from integration submits all tip stacks #1257
                  • integration: tip clean prunes subsumed tips #1258
                  • integration: tip advance moves tips to upstack leaves #1259
                  • integration: rebuild fixes and reliability improvements #1260

_{Change managed by git-spice.}