Give a second checkout its own build output

Two worktrees sharing one cargo target get one artifact hash, and a plain
cargo test in the older checkout ran the other's mutant binary. Found by
Soul on the Huginn Cut 10 second fix batch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

SKILL.md

@@ -389,6 +389,14 @@ and reconciles the target doc with the Body.
   inserted carriage returns against the worktree, which poisons every byte
   count built on it. Read raw blobs and verify against the worktree's hash
   before comparing anything.
+- **A shared build output directory belongs to one checkout of a repository.**
+  Two worktrees of the same crate get the same artifact hash. Cargo decides
+  whether to rebuild by comparing modification times, so a plain `cargo test` in
+  the checkout with older sources ran the other checkout's last mutant binary. A
+  surviving mutant can make an unmutated tree look green this way. A second
+  checkout, such as Soul's clone or a parallel Hands worktree, gets its own
+  target subdirectory. The mutation harness is immune because it bumps the time
+  on every file it writes. A plain test run is not.
 - **Write commit messages with the Write tool to a uniquely named scratch file,
   then `git commit -F`.** PowerShell 5 here-strings break `-m` quoting, and its
   `Out-File`/`Set-Content` write a BOM into message files.

references/changelog.md

@@ -2,6 +2,43 @@
 
 Record each change to the skill together with the evidence that motivated it.
 
+## 2026-09-22: a shared cargo target can run another checkout's mutant
+
+Evidence: Soul, working on the Huginn Cut 10 second fix batch, had two worktrees
+of the same repository. Both built into `C:\Users\Meta\.cargo-target-codex`, and
+both produced the same artifact name, `huginn_daemon-e1f33b5708db9f04.exe`.
+A plain `cargo test` in the second worktree did not rebuild, because its sources
+were older than that binary. It ran the first worktree's last harness mutant
+instead: 40 runs out of 40 failed at the mutant's assert line, not at the
+source's.
+
+- The case that does damage is the reverse. A surviving mutant's binary makes an
+  unmutated tree look green.
+- The harness is immune, because every write it makes, M0's included, bumps the
+  file's modification time. A plain test run in a second checkout gets no such
+  protection.
+- Rule, in SKILL.md's git and tooling list: a shared build output directory
+  belongs to one checkout per repository. Any second checkout gets its own
+  target subdirectory.
+
+The same evening, Self briefed four agents freehand, and none of the briefs
+carried the template's wait paragraph. That breaks the 2026-09-17 rule below.
+Self sent the paragraph to the three agents that could still act on it. The
+rule stands. What failed was Self's own compliance with it.
+
+## 2026-09-17: Self's own briefs dropped the template's wait rule
+
+Evidence: in the Aetheria item-provenance session, two Hands passes (the Cut C census
+follow-up and the neutral wanderers and IFF patch) ended their turns while a mutation run
+was still in the background. Each had to be resumed to finish, push and clean up.
+
+- The Hands template in `briefs.md` already says a wait is a foreground poll, and that a
+  detached job cannot wake an agent whose turn has ended.
+- Self wrote those briefs freehand and left that paragraph out. The defect was in Self's
+  briefing, not in the template.
+- Rule: when Self briefs Hands or Soul without pasting the template, it still carries the
+  template's wait paragraph verbatim.
+
 ## 2026-09-17: three passes to pin one line, because each proved a cousin
 
 Evidence: the QUIC bridge's "the wait is the timeout the host asked for", over
@@ -397,3 +434,17 @@ evidence is `postmortem-cultcache.md` and the Aetheria repo's
 - **Git and tooling scars:** explicit staging, message files, the 3-tag limit,
   commit-independent byte checks, detached builds, and the process-probe role
   check.
+
+- 2026-09-18: the wait rule now names the failure shape, not only the
+  mechanism. Hands ended two turns announcing it would wait for a mutation
+  suite (Aetheria shield Cut 3), the second after an explicit correction. The
+  brief already said how to wait; it did not say that saying "I'll wait" and
+  yielding is the violation. Both copies in `briefs.md` now do.
+
+- 2026-09-18: context is a budget. The operator noticed Sonnet Hands runs
+  passing 400k tokens on Aetheria's stats and shield cuts and said they "start
+  to get loopy with that much context" — matching where the sloppy claims in
+  those runs appeared. `briefs.md` now tells Self to size cuts to one worker's
+  head: verify once at the end, quote the cut inline instead of pointing at the
+  whole map, keep expensive scaffolding alive across cuts, and hand back a
+  clean partial rather than push through.