feat(enterprise): add migration phases

[Gracker]

Summary

• add an enterprise migration plan for legacy, P-A dual-write, P-B cutover, and P-C retired phases
• route trace/report/provider/runtime snapshot and RAG/baseline/case/project-memory stores through phase-aware read authority and write targets
• add dry-run, filesystem snapshot, SQLite DB snapshot, and snapshot restore support with a CLI entrypoint
• cover phase semantics, trace dual-write/cutover behavior, and snapshot restore; mark README §0.3.8 and §0.6.9 complete

Verification

• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run typecheck (backend)
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/services/tests/enterpriseMigration.test.ts (backend)
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/services/tests/enterpriseMigration.test.ts src/services/tests/enterpriseKnowledgeScope.test.ts src/services/providerManager/tests/enterpriseProviderStore.test.ts src/routes/tests/enterpriseTraceMetadataRoutes.test.ts src/routes/tests/enterpriseReportRoutes.test.ts src/services/tests/runtimeSnapshotStore.test.ts src/agentv3/tests/claudeRuntimeRuntimeSnapshots.test.ts src/agentv3/tests/analysisPatternMemory.test.ts src/services/tests/ragStore.test.ts src/services/tests/baselineStore.test.ts src/agentv3/tests/projectMemory.test.ts src/services/tests/caseLibrary.test.ts src/services/tests/caseGraph.test.ts (backend)
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:core (backend)
• git diff --check
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:scene-trace-regression (backend)
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run enterprise:migration -- --dry-run --snapshot-dir /snapshots (backend, temp env)

[Gracker]

已通过 commit 67856ec (merge) + 023a84a (frontend rebuild) 合入 main，关闭此 PR。