docs(agents): stacked PR series are not a supported working form

Record the maintainer's ruling (director seat, decision batch #66): a
stacked series -- each PR branched off the one below -- is not a supported
working form in this repository, and no gate rule or merge-policy change is
made for it. A multi-card change uses a trunk branch and pays the two
recorded workarounds.

The paragraph is self-contained rather than a pointer: AGENTS.md is in
check:pm-skill-id-lint's scan set (pattern /#[0-9]{3,}/), so the card number
cannot be cited in the file. The three tooling blind spots are stated in one
clause each so the rule is actionable without dereferencing history.

Fold payment for the 1068/1068 line ratchet: the ADR-0087 marker block in
the Post-Task Checklist listed 4 of the gate's 7 disposition categories --
a drifted copy of output the same paragraph already calls "the authority".
Replaced by a pointer to the gate's own FIXIT, which prints the full set.
Net 0 lines.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P58euzUXCVJNwmhuPC9DXY

Author: claude

AGENTS.md

@@ -461,6 +461,13 @@ Even inside your own worktree, operate defensively:
    The issue number is what makes in-flight work *discoverable* — `git ls-remote --heads
    origin | grep issue-<n>` is a one-command pre-check, and the Duplicate Fix Guard
    workflow warns on fix PRs whose branch names no declared issue.
+
+   ⛔ **Off `main` is literal — a stacked series, each PR branched off the one below, is NOT a supported form.**
+   Nothing in the tooling represents it: heavy workflows trigger on `main` only, so a stacked head reports NONE of
+   §7's six required contexts; squash landing destroys the ancestry link, so every descendant rewinds behind what
+   landed and pays a rebuild lap per landing; and a breaking changeset's ADR-0087 disposition is base-relative, so a
+   stacked card's two bases demand contradictory markers. A multi-card change uses a **trunk branch**: correct the
+   trunk's disposition to `registered` before it merges, and pay the rebase laps. ⛔ No gate or merge-policy change.
 3. **Never `git push --force` / `--force-with-lease`, and never push `main`.** A
    force-push can clobber a parallel agent's work; `main` is shared — land all via PR.
 4. **Verify the current branch before every commit/push**
@@ -1037,16 +1044,9 @@ registry? Add it to `OPEN_CAPABILITY_REGISTRIES` in the same PR that fixes it.
    schema is `.strict()`. The changeset is one of fourteen surfaces a retirement touches — follow the
    `spec-property-retirement` skill (`.claude/skills/`) rather than reconstructing the kit, and note the two routes
    imply **opposite** liveness-ledger dispositions.
-   **A breaking changeset must also state its ADR-0087 disposition, in writing.** Add exactly one marker to the
-   changeset body — `pnpm check:adr-0087-registration` enforces it, and the CI step is *Require an ADR-0087
-   disposition on a declared-breaking changeset*:
-   ```
-   <!-- adr-0087: registered SOME-MIGRATION-ID -->
-   <!-- adr-0087: not-required (unpublished) why -->
-   <!-- adr-0087: not-required (already-registered SOME-MIGRATION-ID) why -->
-   <!-- adr-0087: not-required (no-migration-prescription) why -->
-   ```
-   The gate prints the argument when it fails — that output is the authority.
+   **A breaking changeset must also state its ADR-0087 disposition, in writing** — exactly one marker in the
+   changeset body, enforced by `pnpm check:adr-0087-registration` (CI step *Require an ADR-0087 disposition on a
+   declared-breaking changeset*). ⛔ The categories are NOT copied here — the gate prints the full set when it fails.
 4. **A removal that breaks the pinned sibling checkout ships together with the sibling fix and the pin bump — or it
    does not ship.** The `Console Pin Gate` job builds objectui at the pinned `.objectui-sha` against **current** `main`,
    so a removal or rename the pinned sibling still imports turns `main` red for every PR in the repo the moment it