Decide where reference implementations are still written

From M3a: a full reference for the audit chain and the approval
handoff, the property test's oracle for policy, and compiling
skeletons for everything else. Each plan records what its checks
exposed, and that record decides what the next milestone gets.
Tip T17; the spec's handover section says which check each module has.

Also restores three / examples that an editing tool had decoded
to a plain slash.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/decisions.md

@@ -6,7 +6,8 @@ Newest first. A decision that changes `docs/design.md` lands in the same commit
 
 | Date | Decision | Reason |
 |---|---|---|
-| 2026-09-18 | Spec review of M3a, approvals. The approval block in `bxctl chat` is fetched from `brokerd` by id, never taken from `loopd`'s event. It shows the parsed arguments serialised again, with control, invisible and direction-changing characters as `\uXXXX`. The owner types the approval id, not `y`. `bxctl chat` escapes model text the same way. A `tool_denied` turn event carries the reason to the owner. | A compromised `loopd` must not choose what the owner approves; `/etc` and `/etc` must look alike; U+202E can make a path read as another; a line already waiting in stdin must never approve; `chat` wrote model text raw, so a conceal sequence could hide the block. |
+| 2026-09-18 | From M3a, a full reference implementation is written only where it still pays: the audit writer and `ChainVerifier` (which also generates the tampering fixtures) and the approval handoff. `policy` is checked by the naive oracle inside its property test. Everything else gets a skeleton: the task's real signatures with `todo!()` bodies, checked to compile against the given tests. Each plan's README records, per task, which check it had and what defect in the spec, task or tests that check exposed; a category that exposes none across a milestone drops a level (reference, skeleton, nothing). Given tests stay for policy, audit and approvals whatever the record says. | In M1 to M2b the reference checked the tasks, not the implementer: it proved tests could pass and signatures compiled (T16), shared the author's blind spots (T5), and caught none of the implementer's defects, which review and probing found. An unsatisfiable test costs one stop-and-report. The tokens are better spent on review. Where a wrong implementation is an authority bypass, the code's author must not also write its acceptance tests. |
+| 2026-09-18 | Spec review of M3a, approvals. The approval block in `bxctl chat` is fetched from `brokerd` by id, never taken from `loopd`'s event. It shows the parsed arguments serialised again, with control, invisible and direction-changing characters as `\uXXXX`. The owner types the approval id, not `y`. `bxctl chat` escapes model text the same way. A `tool_denied` turn event carries the reason to the owner. | A compromised `loopd` must not choose what the owner approves; `\u002fetc` and `/etc` must look alike; U+202E can make a path read as another; a line already waiting in stdin must never approve; `chat` wrote model text raw, so a conceal sequence could hide the block. |
 | 2026-09-18 | Spec review of M3a, `brokerd` internals. One lock (the ledger) covers the audit writer and the session state files; the pending table has its own, and whoever removes an entry answers it. After any failed audit write, `brokerd` denies with `audit_unavailable` until restarted. `policy` is pure, and `decide` never returns a `Decision` for an `ask` grant: only `redecide` turns an `Ask` into one. A `RunError`'s text is fixed text, never tool output, and a failure does not raise taint. `BrokerPort` waits `[broker] timeout_ms` for a first frame, and until `expires` plus that after a pending one. | Without the lock two results could lower a session's taint. A partial line followed by more records is a broken chain. `loopd` logs failures as `public`, so tool output in one would be unlabelled. |
 | 2026-09-18 | Spec review of M3a, policy. A `deny` grant must have `max_taint = "secret"` or the grant set is invalid. A result's `result_class` and `untrusted` are combined over every matching grant (highest class; untrusted if any says so), and within a mode the grant with the longest matched path wins before the id tie-break. A grant path of `/` is invalid. | A `deny` grant with a lower `max_taint` stopped denying once the session read a secret. With overlapping `auto` grants the id tie-break chose the label, so a secret could come back `private`. M3b mounts grant paths at the same path, where `/` would replace the tool image. |
 | 2026-09-18 | Spec review of M3a, audit. A line followed by a matching `Recovery` is recovered whether or not it parses; an unparseable last line is recovered like a torn one. The writer never moves to an earlier day's file, and `Recovery` and `AcceptedBreak` go in the latest file. `--accept-break` verifies the whole log; the verifier checks an `AcceptedBreak`'s `file`, `line`, `last_good`, `prev` and `seq`, and the `seq` is counted from lines, not read from them. The `Approval` record carries the re-decision's grant and session state. The report lists allowed calls with no `Result`. | As first written, a record cut exactly before its newline made `brokerd` break its own chain; so did a clock stepped back over midnight; a break in an older file could never be accepted; and a damaged line could choose the next `seq`. |