docs: README section on keeping a profile current

Documents that profile slots are versioned with the plugin, not the
project: optional slots degrade to documented no-ops when absent, so
an out-of-date profile silently skips the gates a new slot powers.
Tells maintainers to skim the template's commented blocks and the
schema after a plugin update, and gives a concrete retrofit recipe
for `spec_validation` (the slot the compound-hallucination hardening
added) and the three gates it powers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -62,6 +62,46 @@ Your project carries a small **profile** that fills the slots:
 See `docs/profile-schema.md` for the full schema and
 `templates/project-profile.yml` for a copy-and-fill starting point.
 
+## Keeping a profile current
+
+Profile slots are versioned with the plugin, not with your project.
+A new optional slot lands in `docs/profile-schema.md` and as a
+commented-out block in `templates/project-profile.yml` — but an
+existing profile does **not** gain it automatically. Every
+consuming skill treats a missing optional slot as a documented
+no-op, so an out-of-date profile never breaks; it just silently
+skips whatever the new slot would have enabled.
+
+That silence cuts both ways: a profile written before a slot
+existed will quietly not run the gate the slot powers, and nothing
+flags it. So after pulling a plugin update, skim the commented
+blocks in `templates/project-profile.yml` and the matching sections
+in `docs/profile-schema.md`; any optional section your profile
+lacks is a candidate to retrofit. The slot reference lives with the
+plugin, never in the project — when in doubt, the schema is the
+source of truth.
+
+### Retrofitting `spec_validation`
+
+The `spec_validation.parsers` slot maps a markdown fence label to
+the tool that validates a spec code block of that kind. It powers
+the parse gates that stop a spec from shipping code blocks that do
+not parse against the live tool:
+
+- `brainstorm` Step 7 self-review — the parse-every-block gate
+- `planner` Step 5 self-review — the parse-the-bytes-you-inline gate
+- the `grounding-check` agent's code-block parse pass
+
+A profile written before this slot existed has no `spec_validation`
+section, so all three gates are silent no-ops there. To opt in, add
+the section: map each fence label your specs use (a surface
+language, a JSON-against-schema block, an IR block, …) to its
+`ext` + `cmd`, where `cmd` carries the `{file}` placeholder and
+exits non-zero on a parse failure. A fence label with no entry is
+skipped and documented, never silently trusted. See
+`docs/profile-schema.md` § `spec_validation` for the shape and
+`templates/project-profile.yml` for a commented example.
+
 ## Install
 
 See `INSTALL.md`. In short: clone, run `install.sh`, then drop