docs(extra_json): policy doc for containers.extra_json evolution

New CODEMAPS/container-extra-json.md documents the contract every
source plugin must follow when reading or writing containers.extra_json.
Closes the open architectural question that was tracked in
WORKLOAD_REFACTOR_TODO.md.

Covers:
- Schema position (column default, four write-path normalization
  sites) and ownership model (per-source row keys, current writers).
- Reader rules: tolerate unknown keys via default json.Unmarshal,
  tolerate decode failure where first-class columns suffice.
- Writer patterns: wholesale-overwrite (image source, single-writer
  short-lived rows) vs preserve-unknown-keys (static source, RMW with
  generic-map round-trip). Preserve-unknown-keys is the recommended
  default for new sources.
- Concurrency: SetMaxOpenConns(1) + WAL gives atomic per-row writes
  and consistent reader snapshots, but does NOT serialize multi-
  goroutine RMW — a per-workload sync.Mutex is required for that
  (fenced by TestSaveState_ConcurrentWritesDoNotLoseUpdates).
- What extra_json is NOT for (workload config, cross-source state,
  queryable data, secrets) and a checklist for adding a new field.
- Pointers to every example in tree: image's containerExtra writer/
  reader, static's saveState round-trip, workload_runtime.go's
  decode-and-tolerate consumer.

WORKLOAD_REFACTOR_TODO Container.extra_json question flipped to DONE.
CODEMAPS/INDEX bumped + entry linked.

Reviewer pass (code-reviewer subagent) caught one HIGH factual error
(wrong cross-source consumer claim) and several MEDIUM/LOW drifts;
all addressed inline before commit.

docs/CODEMAPS/INDEX.md

@@ -1,6 +1,6 @@
 # Tinyforge Codemaps — Index
 
-**Last Updated:** 2026-05-16
+**Last Updated:** 2026-05-16 (added `container-extra-json` policy doc)
 
 This directory contains architectural maps of key Tinyforge subsystems. Each codemap focuses on one major area: core data types, contract surfaces, integration points, and recipes for extending the system.
 
@@ -8,6 +8,7 @@ This directory contains architectural maps of key Tinyforge subsystems. Each cod
 
 - **[Workload Plugin](./workload-plugin.md)** — Source × Trigger plugin contracts; registry lookups; webhook fan-out; how to add new kinds.
 - **[Discovery & Runtime API](./discovery-and-runtime.md)** — `/api/discovery/*` helpers (Git provider probe, repo/branch/tree pickers, image conflicts); `/api/workloads/{id}/runtime-state` + `/storage` + `/stop` + `/start`; SSRF-safe HTTP client in `internal/staticsite`.
+- **[`containers.extra_json` Evolution Policy](./container-extra-json.md)** — Ownership model, reader/writer rules, wholesale-overwrite vs preserve-unknown-keys patterns, concurrency invariants; checklist for adding a new field without breaking older deployers.
 
 ## Cross-References