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/plans/M3b/03-brokerd-grant-mount-rule.md

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+# M3b task 03: a grant path that cannot be mounted is invalid
+
+**Branch:** `m3b` (run `git switch m3b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `Refuse grant paths that cannot be mounted`
+
+## Goal
+
+From M3b, a grant's paths are mounted into the tool container as `--volume=<path>:<path>:ro`. In
+that argument `:` and `,` are separators, so a path containing either would be read as something
+else. A grant with such a path is **invalid**, which (as for every invalid grant) makes the whole
+set invalid and denies every call until it is fixed.
+
+## Files
+
+- Copy: `crates/brokerd/tests/grants_mount.rs`
+- Modify: `crates/brokerd/src/grants.rs`, `docs/implementer-log.md`
+
+## The rule
+
+In `check_grant`, the loop over `inner.constraints.paths` has two cases today: the path is `/`, or
+it is not a valid path. Add a third, checked **only when the first two did not apply** (a path that
+is already reported as invalid is not reported twice):
+
+```rust
+} else if path.contains([':', ',']) {
+    // Mounted as `--volume=<path>:<path>:ro`, where both are separators.
+    push(problems, file.clone(), None,
+         format!("{:?} cannot be mounted: it contains ':' or ','", path));
+}
+```
+
+So a grant file with `paths = ["/home/kyle/a:b"]` gives exactly one problem, for that file, whose
+text contains `cannot be mounted`. Other punctuation (space, `;`, `=`, `.`, `-`, `_`) stays fine.
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m3b`, then
+  `cp docs/plans/M3b/files/crates/brokerd/tests/grants_mount.rs crates/brokerd/tests/`
+- [ ] **2. See it fail.** `cargo test -p brokerd --test grants_mount`. Expected: 1 of 2 fails
+  (`a_path_with_a_colon_or_a_comma_makes_the_set_invalid`).
+- [ ] **3. Add the rule.** Run `cargo fmt --all`.
+- [ ] **4. See it pass.** `cargo test -p brokerd --test grants_mount --test grants`. Expected: 2 and
+  17 passed.
+- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
+- [ ] **6. Log and commit.** `git add crates/brokerd docs/implementer-log.md && git commit`
+
+## Done when
+
+- Both suites pass; `make gate` prints `gate: ok`.
+
+## Stop and report if
+
+- A given test wants a path with `:` or `,` to be accepted, or wants two problems for one path.