fix(lanes): keep stage detection honest across rollbacks and feature switches

Three defects let a lane's pipeline map claim progress that was not this
pass's work, all of them on the "going back" path a new request takes:

- `setStage` cleared the detection columns on a real transition but never
  pruned the `stages` records AHEAD of the new stage. A lane that ran through
  `done` and was then declared back at `plan` rendered `plan` as `current`
  while `tests`/`review`/`done` still read `done`, carrying the previous
  pass's evidence, and contradicting `progress` (computed from the declared
  index alone). A backwards declaration now drops every record it can prove
  is ahead; a key matching no node is unplaceable and left alone.
- `activateFeature` rewrote the live lane row for the incoming feature but
  carried the outgoing one's `detected_*` across. A never-seen slug starts at
  `stage='idle'`/`stages={}`, exactly the shape `inferredDisplayStage` renders
  from detection, so the new feature opened painted with the old one's
  progress — and forward-only rejected every real detection for it until the
  carried-over one aged past DETECTION_TTL_MS. Now cleared, like every other
  real transition; deliberately not archived, since detection is live-only.
- `codex-ingest` classified a rollout journal by raw `now - mtime`, which goes
  negative for a future mtime (clock stepped back, NTP correction, a restore
  that preserved timestamps) and read as "written milliseconds ago". A
  finished thread's journal could replay as live work; a file claiming to be
  from the future is now idle.

Also fixes `check-headers.sh`, which audited via `find` and so reported local
gitignored runtime state (a lane's own `.ccam/profile/hooks/*.sh`) that is not
this repo's source and can never be made compliant here. It now lists files
through git (tracked + untracked-but-not-ignored), so a file an agent just
wrote is still caught before it is staged while ignored state is not.

Tests: 1359 pass (4 new). Header audit exits 0.

server/__tests__/lane-features.test.js

@@ -102,6 +102,27 @@ describe("activateFeature / archiveActiveFeature", () => {
     assert.equal(features.getFeature(lane.id, "one").archived_at, null);
   });
 
+  it("switching features clears the outgoing feature's detection", () => {
+    // Without this, a brand-new slug (stage='idle', stages={}) renders its map
+    // from the PREVIOUS feature's detection via inferredDisplayStage, and
+    // forward-only rejects every real detection for the new feature until the
+    // carried-over one ages past DETECTION_TTL_MS.
+    const lane = makeLane();
+    features.activateFeature(lane.id, "one");
+    lanesLib.recordDetection(lane.id, { nodeId: "ship", signal: "`git push`" });
+    assert.equal(lanesLib.getLane(lane.id).detected_stage, "ship");
+
+    const { lane: updated } = features.activateFeature(lane.id, "two");
+    assert.equal(updated.detected_stage, null);
+    assert.equal(updated.detected_signal, null);
+    assert.equal(updated.detected_at, null);
+
+    // ...and a detection for the new feature lands immediately.
+    assert.deepEqual(lanesLib.recordDetection(lane.id, { nodeId: "plan", signal: "`Write`" }), {
+      written: true,
+    });
+  });
+
   it("archiveActiveFeature returns null and touches nothing when no feature is active", () => {
     const lane = makeLane();
     assert.equal(features.archiveActiveFeature(lane.id), null);