fix(learnings): accept type:"investigation" in gstack-learnings-log

The /investigate skill instructed agents to log learnings with type:"investigation",
but bin/gstack-learnings-log:22 rejected anything not in
[pattern, pitfall, preference, architecture, tool, operational]. Every
investigation run exited 1 to stderr and the learning was dropped, silently
to the user.

Fix: add 'investigation' to ALLOWED_TYPES.

Regression test: round-trips a learning with type:"investigation" and asserts
exit 0 + file write; second test reads investigate/SKILL.md.tmpl and asserts
it emits the literal type:"investigation" string, guarding the
template/validator contract at both ends.

Fixes #1423. Reported by diogolealassis.

bin/gstack-learnings-log

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 # gstack-learnings-log — append a learning to the project learnings file
 # Usage: gstack-learnings-log '{"skill":"review","type":"pitfall","key":"n-plus-one","insight":"...","confidence":8,"source":"observed"}'
+# Valid types: pattern, pitfall, preference, architecture, tool, operational, investigation
 #
 # Append-only storage. Duplicates (same key+type) are resolved at read time
 # by gstack-learnings-search ("latest winner" per key+type).
@@ -19,7 +20,7 @@ let j;
 try { j = JSON.parse(raw); } catch { process.stderr.write('gstack-learnings-log: invalid JSON, skipping\n'); process.exit(1); }
 
 // Field validation: type must be from allowed list
-const ALLOWED_TYPES = ['pattern', 'pitfall', 'preference', 'architecture', 'tool', 'operational'];
+const ALLOWED_TYPES = ['pattern', 'pitfall', 'preference', 'architecture', 'tool', 'operational', 'investigation'];
 if (!j.type || !ALLOWED_TYPES.includes(j.type)) {
   process.stderr.write('gstack-learnings-log: invalid type \"' + (j.type || '') + '\", must be one of: ' + ALLOWED_TYPES.join(', ') + '\n');
   process.exit(1);