perf(ci): gate prompt-snapshot regeneration on the generator's blast radius (#109083)

check-prompt-snapshots costs 200-310s per PR regenerating real prompt
stacks, but snapshots only change when the generator's import graph or
its fixtures do. The manifest now computes that reach (same pattern as
QA smoke gating: always-run surface regex incl. the codex extension
whose test API loads via a dynamic module id, plus import-graph walk
from the snapshot helper; deletions and null diffs fail safe to
running). Unaffected PR diffs skip the lane with an explicit log line;
pushes and dispatches always run.

.github/workflows/ci.yml

@@ -104,6 +104,7 @@ jobs:
       run_checks_fast: ${{ steps.manifest.outputs.run_checks_fast }}
       historical_target: ${{ steps.manifest.outputs.historical_target }}
       run_qa_smoke_ci: ${{ steps.manifest.outputs.run_qa_smoke_ci }}
+      run_prompt_snapshots: ${{ steps.manifest.outputs.run_prompt_snapshots }}
       checks_fast_core_matrix: ${{ steps.manifest.outputs.checks_fast_core_matrix }}
       run_plugin_contracts_shards: ${{ steps.manifest.outputs.run_plugin_contracts_shards }}
       plugin_contracts_matrix: ${{ steps.manifest.outputs.plugin_contracts_matrix }}
@@ -554,6 +555,13 @@ jobs:
             eventName !== "pull_request" ||
             typeof changedNodeTestPlan.hasQaSmokeAffectingChange !== "function" ||
             changedNodeTestPlan.hasQaSmokeAffectingChange(changedPaths);
+          // Prompt snapshots only change when the generator's import graph or
+          // its fixtures do; unaffected PR diffs skip the regeneration lane.
+          const changedScopeHasPromptSnapshotImpact =
+            changedPaths === null ||
+            eventName !== "pull_request" ||
+            typeof changedNodeTestPlan.hasPromptSnapshotAffectingChange !== "function" ||
+            changedNodeTestPlan.hasPromptSnapshotAffectingChange(changedPaths);
           const runBuildArtifacts = runNodeFull && changedScopeHasBuildImpact;
           const runQaSmokeCi =
             runNodeFull &&
@@ -614,6 +622,7 @@ jobs:
             historical_target: historicalTarget,
             compatibility_target: compatibilityTarget,
             run_qa_smoke_ci: runQaSmokeCi,
+            run_prompt_snapshots: runNodeFull && changedScopeHasPromptSnapshotImpact,
             checks_fast_core_matrix: createMatrix(checksFastCoreTasks),
             run_plugin_contracts_shards: runPluginContractShards,
             plugin_contracts_matrix: createMatrix(
@@ -2021,6 +2030,7 @@ jobs:
       - name: Run additional check shard
         env:
           ADDITIONAL_CHECK_GROUP: ${{ matrix.group }}
+          RUN_PROMPT_SNAPSHOTS: ${{ needs.preflight.outputs.run_prompt_snapshots }}
           OPENCLAW_ADDITIONAL_BOUNDARY_SHARD: ${{ matrix.boundary_shard || '' }}
           OPENCLAW_ADDITIONAL_BOUNDARY_CONCURRENCY: 4
           OPENCLAW_EXTENSION_BOUNDARY_CONCURRENCY: 6
@@ -2051,8 +2061,14 @@ jobs:
             prompt-snapshots)
               # No presence fallback: the boundary runner previously invoked
               # this unconditionally, and silent success would drop snapshot
-              # drift coverage.
-              run_check "prompt:snapshots:check" pnpm prompt:snapshots:check
+              # drift coverage. The manifest gates the lane on the generator's
+              # import graph and fixtures; diffs outside both cannot change
+              # generated snapshots.
+              if [ "$RUN_PROMPT_SNAPSHOTS" != "true" ]; then
+                echo "[skip] changed scope cannot affect generated prompt snapshots"
+              else
+                run_check "prompt:snapshots:check" pnpm prompt:snapshots:check
+              fi
               ;;
             session-accessor-boundary)
               if [ ! -f scripts/check-session-accessor-boundary.mjs ]; then