docs: clarify capture_thought is an optional convention (#1873) (#2379)

* docs: clarify capture_thought is an optional convention (#1873)

Issue #1873 merged /gsd:extract-learnings with an optional
capture_thought hook, but the docs never explained what the tool is
or where it comes from — readers couldn't tell whether it was a
bundled GSD tool, a required dependency, or something they had to
install. This surfaced in a user question on that issue's thread.

Clarify in docs/FEATURES.md §112 and the workflow file that
capture_thought is a convention — any MCP server exposing a tool
with that name will be used; if none is present, LEARNINGS.md
remains the primary output and the step is a silent no-op.

No behavioral change. All 23 extract-learnings tests still pass.

* fix(security): add human to detection message; test [/INST] closing form neutralization

- Detection message now lists <human> alongside <system>/<assistant>/<user>
- Sanitizer regex extended to cover [/INST] closing form (was only [INST])
- Detection pattern extended to cover [/INST] closing form
- New sanitizeForPrompt test asserts [/INST] is neutralized

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(config): add workflow.security_* keys to VALID_CONFIG_KEYS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: add language tag to fenced code block in FEATURES.md

Fixes MD040 lint finding in PR #2379 — the capture_thought tool
signature example was missing a javascript language identifier.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Tom Boucher <trekkie@nomorestars.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/FEATURES.md

@@ -2371,6 +2371,20 @@ Test suite that scans all agent, workflow, and command files for embedded inject
 
 **Produces:** `{phase}-LEARNINGS.md` with YAML frontmatter (phase, project, counts per category, missing_artifacts)
 
+**Optional integration — `capture_thought`:** `capture_thought` is a **convention, not a bundled tool**. GSD does not ship one and does not require one. The workflow checks whether any MCP server in the current session exposes a tool named `capture_thought` and, if so, calls it once per extracted learning with the signature below. If no such tool is present, the step is skipped silently and `LEARNINGS.md` remains the primary output.
+
+Expected tool signature:
+```javascript
+capture_thought({
+  category: "decision" | "lesson" | "pattern" | "surprise",
+  phase: <phase_number>,
+  content: <learning_text>,
+  source: <artifact_name>
+})
+```
+
+Users who run a memory / knowledge-base MCP server (for example, ExoCortex-style servers, `claude-mem`, or `mem0`-style servers) can implement this tool name to have learnings routed into their knowledge base automatically with `project`, `phase`, and `source` metadata. Everyone else can use `/gsd-extract-learnings` without any extra setup — the `LEARNINGS.md` artifact is the feature.
+
 ---
 
 ### 113. SDK Workstream Support