Specify and plan M3b: the runner and the tools

A draft spec for the owner's review and 13 offline tasks with their given
tests: shared tool arguments and host rules in proto, the sealed fetch
target (M3a finding 14), the toolkit tools and SOCKS5 egress proxy, and
brokerd's [runner], podman argument lists, runtime and proxy lifecycle. Each
task's tests were run against a reference at that task's end state (560 to
638 tests, clippy clean); the reference is not in the repository. Adds the
runner-unavailable runbook entry and tip T23 (ETXTBSY in script tests).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/decisions.md

@@ -6,6 +6,7 @@ Newest first. A decision that changes `docs/design.md` lands in the same commit
 
 | Date | Decision | Reason |
 |---|---|---|
+| 2026-09-22 | M3b spec (`docs/specs/2026-09-22-m3b-runner.md`), draft for the owner's review, written for an overnight run. New: the tools' argument structs and the host rules move to `proto`, shared by `brokerd` and `toolkit`; `toolkit` exits 0/1/2 (done / the tool's own error, shown to the model / misuse, a fixed sentence); the egress proxy refuses any allowed name that resolves to a non-public address (including the tailnet's `100.64/10` and `::/96`); grant paths may not contain `:` or `,`; `[runner]` names the image by digest; tool time limits sit under `loopd`'s 120 s broker timeout, so `shell` defaults to 100 s. | A grant allows a host on the internet, not what its name points at; two programs reading the same data need one definition; `--volume` uses `:` and `,` as separators. The shell limit follows from the M3a deadline in `BrokerPort`. |
 | 2026-09-22 | M3a review fixes. `loopd` waits at most a day after a pending frame and `brokerd` refuses `[approvals] ttl_ms` over a day. A socket whose directory is `/` or a symbolic link is refused at start. A listener out of file descriptors or memory pauses and retries instead of stopping `brokerd`. `brokerd` and `bxctl` share one rule for audit log file names (`proto::is_audit_log_name`, real months and days only). The fixes were made by the design model, and the fix commits were reviewed by a separate agent before the merge. | A far `expires` parked a turn for ever; `chmod` through a link changed its target; an idle-connection flood could stop the daemon; the two components disagreed about which files were the log. Ornith was under heavy contention. |
 | 2026-09-18 | M3a plan checks, `brokerd`. Every tool request gets a `Decision` record, including those denied `grants_invalid` or `state_unreadable`; forbidden kinds get none. An unreadable session state is recorded as `secret`, untrusted. A refusal that cannot be recorded is `error internal`, not `ok`. The re-decision's outcome is the matched grant's mode. A pending frame that cannot be sent is handled like a lost connection. Expiry lives in `admin`. A request frame without a read timeout is accepted for M3a. | Found while writing the reference for tasks 10 to 15: the spec left each case open, and `bxctl refuse` would have reported success for a refusal that was not on disk. |
 | 2026-09-18 | M3a plan checks, audit. `DecisionRecord::Allowed {}` and `Ask {}` are empty struct variants. The resumed verifier accepts a break naming an older file inside a failed region too. When the previous file's last line does not parse, an ordinary start verifies the whole log. A `Recovery` that describes no line is a failure. `abandoned` and `unfinished` both name the decision's `seq`. The report carries what a `Recovery` or `AcceptedBreak` must hold. `--accept-break` with nothing to accept writes nothing. | serde ignores `deny_unknown_fields` on unit variants of an internally tagged enum, so `{"outcome":"allowed","x":1}` decoded; found by `strict.rs` in two areas, and the struct-variant fix was kept over a hand-written `try_from` as the smaller one that also refuses `"reason":null`. As specified, a correctly accepted break could stop every later start while `--accept-break` said "nothing to accept". |