CHE-18: support daemon-local Git repository sources

[cheruvian]

Summary

• accept daemon-scoped absolute file URLs on existing github_repo project resources
• canonicalize and validate local Git repositories on the owning daemon before using the existing cache/private-checkout pipeline
• allow agents to push task branches back to local repositories without modifying the canonical checkout
• hide machine-local paths from other daemons and support file URLs in the project UI and CLI

Verification

• go test ./internal/handler ./internal/daemon ./internal/daemon/repocache
• go test ./cmd/multica -run 'TestBuildResourceRef|TestRunRepoCheckout'
• pnpm typecheck
• end-to-end regression creates two concurrent isolated checkouts from a local-only commit, pushes both task branches, and verifies the canonical HEAD and dirty state are unchanged

Closes CHE-18

[cheruvian]

Closing this automated upstream PR. Work will stay in the private repository; promotion to a public/upstream PR is a manual human decision.