feat: mirror assistant project effects through Cairnline seams

[chicoxyzzy]

Summary

• split Project Assistant project-level apply mirrors across metadata, root, and defaults seams
• keep proposal-record mirroring from replacing Cairnline-owned roots or sources
• add a regression test covering assistant metadata/default/root side effects while preserving Cairnline-only graph state

Tests

• GOCACHE="$PWD/.gocache" go test ./internal/api -run 'TestProjectAssistantAPI_ProjectSideEffectsMirrorThroughNarrowCairnlineSeams|TestProjectAssistantAPI_ApplyMirrorsCoordinationGraphToCairnlineWhenConfigured|TestProjectAssistantAPI_PartialApplyMirrorsCommittedActionsToCairnlineWhenConfigured|TestProjectsAPI_(MetadataOnlyPatch|RootListPatch|ContextSourceListPatch)'\n- GOCACHE="$PWD/.gocache" go test ./internal/cairnlinebridge\n- GOCACHE="$PWD/.gocache" go vet ./internal/api ./internal/cairnlinebridge\n- just agent-docs-check\n- git diff --check\n- GOCACHE="$PWD/.gocache" go test ./internal/api\n- GOCACHE="$PWD/.gocache" go test -race -timeout 10m ./...\n\n## Docs\n- No docs changed; existing Cairnline assistant mirror seam docs still describe the public behavior.