feat(610): split the decision corpus into one YAML-frontmatter file per record

168 records -> docs/decisions/records/<area>/<topic>.md (163 active, 23 dirs) and
docs/decisions/archive/<area>/<topic>.md (5 archived). The filename IS the key,
so one-active-record-per-key becomes a filesystem property rather than a
validator check, and supersession becomes a `git mv`.

WHY: the monolith was a concurrency problem before an aesthetic one. A
3,900-line append target made parallel sessions collide -- PR #605 and PR #614
both hit append-vs-append conflicts during routine rebases, and hand-resolving
those inside the corpus is exactly the operation the rationale-rewrite guard
exists to police.

HOW IT IS VERIFIED: a ~170-file diff cannot be meaningfully read, so correctness
does not rest on reading it. The parser was taught BOTH formats first, so the
body-diff guard parses the old form at the merge-base and the new form at head --
the migration validates itself, no bypass. The proof is a field-level equivalence
harness: 168 records before and after, zero lost, zero gained, zero field
mismatches, zero rationale bodies differing. Reviewers should scrutinise the
harness; it is the actual evidence.

What measuring caught that reading would not have:

- ~500 lines sit OUTSIDE any record -- decisions.md's lifecycle schema and each
  topic file's preamble, mostly the only copy. Source files are kept and
  stripped, never deleted. They also cannot be filed per-area: topic files hold
  several areas and 4 of 23 areas span several files.
- Archive discovery was a non-recursive glob; after the split it found ZERO
  archived records, surfacing as four bogus "supersedes points to unknown key"
  errors rather than an obvious failure.
- ~32 live docs point into the corpus BY DATE, which the split dangles. Each
  stripped file now ends with a generated "Records formerly in this file" index,
  which also rescues the identical breadcrumbs in old issue comments.
- decisions.md's "In this file:" list was 97 same-file anchor bullets that the
  split makes WRONG, not merely stale. Dropped; the generated index replaces
  them with links that resolve.

The equivalence harness now runs against a checked-in FIXTURE, not the live
corpus. The earlier version migrated the real tree, which made it a one-shot:
the moment the migration landed there was nothing left to move and the tests
failed for reasons unrelated to the code. A fixture keeps them testing the
SCRIPT rather than the repo's current state.

Keys preserved verbatim, warts included: `sched` (12) and `scheduling` (1) remain
two directories for one concept. Renaming a key is not a move -- it changes
identity, breaks the equivalence proof, and invalidates MemPalace's per-key
drawers. Taxonomy normalisation is separate work.

refs #610

docs/decisions/records/session/shared-checkout-refresh.md

@@ -0,0 +1,34 @@
+---
+key: session.shared-checkout-refresh
+title: 2026-07-21 — Session end fast-forwards the shared checkout; a stale tree serves stale FILES (#541)
+status: active
+since: '2026-07-21'
+supersedes: none
+superseded-by: none
+rule: Session end runs `scripts/refresh-shared-checkout.sh`, which fast-forwards `/Users/timothy/ersatztv` to `origin/main` (and reinstalls `web/node_modules` when the lockfile moved), refusing to touch anything unless that tree is on a clean, non-ahead `main`.
+signals: 'shared checkout, stale kickoff paste, `/Users/timothy/ersatztv`, session-end protocol, H13 · paths: `scripts/refresh-shared-checkout.sh`, `docs/handoffs/chicorytv-issue-queue.md` · issues: #541, #520, #311, #312'
+mechanics: '`docs/handoffs/chicorytv-issue-queue.md` → session-end step 6 + the shared-tree lore bullet'
+---
+
+**The existing rule had a hole, and it is a hole no rule can close.** The standing guidance — never
+commit in the shared tree, never read its `HEAD`/`git log`/`git status` as truth about `main` — is
+written entirely around a session **reading git state**. On 2026-07-21 the trap arrived as a **file**:
+the kickoff prompt was pasted out of that tree while it was 81 commits behind, and the handoff doc it
+carried still described the queue protocol #520 had retired the previous day (read tracker #237, which
+`main` now says must *not* be read for queue state). No `git` command touched that tree all session, so
+no discipline check could have fired. Selection happened to go through `scripts/select-queue.sh`, which
+is why nothing broke — routing luck, not a control.
+
+**So the fix removes the stale condition rather than adding a check**, which is what the shared-tree
+lore bullet already prescribed for its first two failure modes: *"a check does not stay true."*
+
+**The script is deliberately timid, because the tree is shared.** It refuses — loudly, exit 0,
+changing nothing — when the tree is not on `main`, is dirty, has local commits, or is mid-rebase or
+mid-merge. It never switches branches, never stashes, never discards. A refusal is a normal outcome,
+not a failure, because the common reason for one is that another session is legitimately mid-flight.
+
+**Two details that testing forced.** The first version used `npm install`, which **rewrote
+`package-lock.json`** and left the shared tree dirty — the exact state the next run refuses on, so the
+tool would have disabled itself after one use. It uses `npm ci`, which installs strictly from the
+lockfile and never writes it. And it asserts the tree is clean at exit, reporting loudly if not:
+leaving the shared tree dirty is the one outcome that would make this script a net negative.