Keep each session's taint and untrusted flag in a file

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

docs/implementer-log.md

@@ -6,6 +6,7 @@ reviewer adds findings under "Reviews" once per milestone.
 
 | Task | Date | Status | Gate runs | First gate | Deviations | Notes | Model |
 |---|---|---|---|---|---|---|---|
+| M3a/08-brokerd-state | 2026-09-18 | done | 1 | pass | none | Wrote crates/brokerd/src/state.rs: RUNBOOK, StateError (Unreadable/Write with hand-written Display ending in RUNBOOK and std::error::Error), StateStore (new does not touch disk, path joins <dir>/<id>.json, read, raise) and the private StateFile with deny_unknown_fields. read has exactly one default path (ErrorKind::NotFound); Public taint is Unreadable; raise computes max(taint,label,Private) and ORs untrusted, always writes atomically in six steps mapping any error to Write(path, err). `cargo fmt` put `state` after `runner` in lib.rs. 9 tests pass; `make gate` prints `gate: ok`. | Laguna S 2.1 |
 | M1/01-workspace-and-gate | 2026-09-17 | done | 1 | pass | none | Crate skeletons, Cargo files and the given Makefile/deny.toml/test-gate-scripts.sh were already present untracked from a prior attempt; I verified them against the plan and created only the missing gate scripts, dependencies.md, egress.md and this log row. | Laguna S 2.1 |
 | M1/02-proto-values | 2026-09-17 | done | 1 | pass | none | Implemented ValueError, SessionId, Epoch, CallId, Hash32 and Timestamp in crates/proto/src/ids.rs and DataClass in class.rs, using serde try_from/into for string-backed JSON validation, a hand-written hex encoder and humantime for RFC 3339 parsing with canonical re-serialization. | Laguna S 2.1 |
 | M1/03-proto-wire | 2026-09-17 | done | 2 | pass | none | Added Envelope, Message, WireError, ErrorCode, ToolRequest, ToolResponse and DenyReason in crates/proto/src/wire.rs, re-exported from lib.rs; all 9 fixture tests pass and `make gate` prints `gate: ok`. | Laguna S 2.1 |