Add the audit writer with its startup check

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

docs/implementer-log.md

@@ -50,6 +50,7 @@ reviewer adds findings under "Reviews" once per milestone.
 | M3a/04-brokerd-config | 2026-09-18 | done | 1 | pass | none | Wrote crates/brokerd/src/config.rs: Paths (Default: home is $BOXMAKER_HOME via var_os else /var/lib/boxmaker, grants /etc/boxmaker/grants), Sockets (derived Default), Approvals (Default ttl_ms 900_000) and Config (derived Default), all with serde(deny_unknown_fields, default) and Config at top level; hand-written ConfigError Read/Parse with Display and std::error::Error; parse/load/broker_socket/admin_socket/audit_dir/state_dir. Added serde, serde_json, toml to Cargo.toml, `pub mod config;` to lib.rs, and `brokerd` to the serde and serde_json "Used by" cells in dependencies.md. 7 config tests pass; `make gate` prints `gate: ok`. | OpenCode |
 | M3a/06-brokerd-grants | 2026-09-23 | done | 2 | fail | none | Wrote crates/brokerd/src/grants.rs: `RUNBOOK`, `LoadedGrant`, `GrantSet` (private `grants` field, `from_grants` sorts by id and collects every problem, `grants()`), `valid_id`, `load` (read_dir -> one directory problem, a missing dir is not empty, sorted names, skip non-.toml, read/utf8/toml/sha256 each record a problem and continue, then `from_grants`, stable sort by file), `render`, and `span_line` (count newlines in `text.get(..offset)` + 1). Rules 2-9 live in `check_grant`/`check_tool_constraints`; an unknown tool skips rule 6 only. First gate failed on clippy `needless_borrows_for_generic_args` (pass `format!()` not `&format!()` to the `impl Into<String>` `push`); all 17 grants tests pass; `make gate` prints `gate: ok`. | OpenCode |
 | M3a/05-brokerd-args | 2026-09-23 | done | 4 | fail | none | Wrote crates/brokerd/src/args.rs (MAX_PATH, MAX_URL, ToolName with ALL/parse/as_str, ToolArgs with tool/canonical_json, ArgsError with hand-written Display+Error, parse, valid_path, inside, valid_host, valid_host_pattern, host_matches, url_host) and added `pub mod args;` to lib.rs. 13 args tests pass; `make gate` prints `gate: ok`. Three clippy fixes before a clean gate: collapsed the shell `cwd` if-let into an edition-2024 let-chain, `('a'..='z').contains` -> `is_ascii_lowercase`, and the trailing `/` match -> `?`. `source()` returns None because `String` does not implement `std::error::Error`. The URL rules read the host as written (no `to_lowercase`); uppercase fails `valid_host`, matching the test that lists `https://Example.com/` as invalid. | OpenCode | Wrote crates/proto/src/chain.rs: `ChainVerifier`, a pure line-holding state machine (each line is judged only once the next one has arrived, so a `Recovery` record can mark the line before it not-a-record), plus `ChainFailure`, `TornTail`, `ChainReport`, `Location`. Holds each line, checks recovery against the next, then rule 1 (parse, expected seq, prev with the file-before text for line 1 of a resumed/continued verifier), the failed-region counting of rule 5, the resumed-earlier-file break exception of rule 6, run/ask tracking for `abandoned`/`unfinished`, and clock warnings; `finish` reports the torn tail and the break's required seq/prev. Added `pub mod chain` and the five re-exports to lib.rs and the same line to audit.rs. The single worker subagent for this task entered an unrecoverable reasoning loop on the state machine and was not completing, so the orchestrator implemented it directly from the spec and fixtures. 13 chain tests pass; `make gate` prints `gate: ok`. | OpenCode |
+| M3a/09-brokerd-audit-writer | 2026-09-19 | done | 2 | fail | `write_record` opens with `.append(true)` (task says "for write") because this environment's `tmpfs` truncates on `write(true).create(true)`; `open` tolerates an already-existing dir (the `case` fixtures pre-create it); `Lock(fs::File)` wrapper added so `Writer` can `#[derive(Debug)]` (the copied tests call `unwrap_err`). | Wrote `crates/brokerd/src/audit.rs`: `Writer`, `Opened`, `AuditError` (Locked/Broken/NothingToAccept/Io/Stopped, hand-written Display ending in the task's RUNBOOK anchors), `verify_dir` (the short check for 2+ files, else full), and `RECOVERED_NOTICE`; `pub mod audit;` in lib.rs. Copied three test files byte-identical. The day-boundary and failed-write tests failed for two real reasons: the appends were silently losing every second line because `tmpfs` truncates on `write(true)` (fixed with `.append(true)`), and the second writer was not being marked `Stopped` after a failed write (fixed per append rule 5). All 16 tests pass (9 audit + 7 audit_startup) across five runs; `make gate` prints `gate: ok`. Two clippy fixes before a clean gate: collapsed the dir-builder `if let` into a let-chain, and added `.truncate(false)` to the lock's open. | OpenCode |
 
 
 ## Reviews