docs(agents): one workspace per agent run — the shared checkout is off-limits

Agents have been working in the operator's checkout and clobbering each other.
Nothing in the tree told them not to: worktrees appeared in CLAUDE.md only as
§3's gate workarounds, and decisions/README.md described worktree isolation as
something agents already do rather than as a rule.

New CLAUDE.md §0 states the rule, with the commands and an explicit do-not list
for the shared checkout. Placed before §1 so every existing §-reference in the
repo keeps its meaning.

Measured in the shared checkout on 2026-10-05, each row a distinct failure mode:
107 `checkout: moving` reflog entries; sitting on feat/pm-agent-issue-to-pr, not
main; two other runs' untracked trees left in it; local main two commits behind
origin/main; 42 registered worktrees of which 31 are on already-merged branches;
9 gate scripts carrying a `.claude/` exclusion because an in-tree worktree made
find-based discovery count the same manifest up to 18 times.

Corrects a claim carried in earlier briefs: `git worktree remove` is NOT blocked
by the agent sandbox. Provoked on a throwaway worktree — rc 0.

§3 reframed as the normal path and grown from two breakages to three:
spec-immutability's skip is avoidable, not merely tolerated, with the spec repo
mounted read-only in the same ENGINE shim plus -e SPEC_REPO=/home-network-k3s.

CLAUDE.md is a sensitive path, so this PR needs a non-author review or an
administrator /approve-override posted as a PR review. Not posted by me: §1
forbids an agent clearing that on its own PR.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

decisions/2026-10-05-agents-work-in-their-own-workspace.md

@@ -95,7 +95,19 @@ post-commit run. §3 now records both expected skips, because it previously name
 - **GATE: UNVERIFIED at the time of writing.** The pinned-toolbox result for this branch and
   its exit code are reported in the PR, not predicted here. **A local rc 3 is not green** and
   will not be described as one.
-- **This PR is a sensitive-path PR and will be CI-RED on `approval-policy` until a human acts.**
+- **CI gate: RED, measured — `MERGE GATE: BLOCKED`, one failing stage.** Run 245 on PR #56,
+  head `b988da8b`, 1m47s. The only failure is `approval-policy`: *"a sensitive change lacks a
+  non-author approval"*, `sensitive paths touched: CLAUDE.md`, author `gitea-admin`. Every other
+  stage passed — `decisions-log`, `lint`, `l0`, `render-golden`, `l1`, and `gitea-settings`
+  (which can only run in CI, and reported `required_approvals = 0`,
+  `dismiss_stale_approvals = true`). `spec-immutability` SKIPPED in CI (no sibling spec repo in
+  the CI workspace — the inverse of the local run, where the two-mount shim made it pass) and
+  `conformance` SKIPPED. **Note `gate / conformance (pull_request)` reports status `success`
+  with description "Has been skipped"** — CLAUDE.md §2b's trap, live on this very PR. Nothing
+  in this branch is broken; the red is the structural human gate, below. *Measured at
+  `b988da8b`; this entry was then amended to record it, so the head SHA in the PR is newer than
+  the run quoted here and CI re-ran on the amended commit.*
+- **This PR is a sensitive-path PR and is CI-RED on `approval-policy` until a human acts.**
   `CLAUDE.md` is in `infra/gitea/approval-policy.json`'s `sensitive_paths` ("the delivery
   contract agents follow"), so the stage requires an approving review from someone who is not
   the author, or an `/approve-override <reason>` posted by an administrator **as a PR review