Add capture/triage inbox loop; dogfood AGENTS.md/ROADMAP; document git-tracking standard

Introduce the cross-project capture->triage->roadmap loop: /capture appends an
idea or bug to INBOX.md from any repo (new-project ideas included), /triage drains
a project's items into its AGENTS.md or ROADMAP.md. Give the standards repo its own
AGENTS.md (+ CLAUDE.md symlink) and ROADMAP.md so it follows its own standard, and
add a 'What git tracks' section to portability.md plus the canonical .gitignore
block answering what is committed vs gitignored around .claude and symlinks.

.gitignore

@@ -1 +1,11 @@
+# Secrets & local env
+.env
+.env.*
+!.env.example
+
+# Claude Code — commit shared config, ignore personal/local
+.claude/settings.local.json
+.claude/*.local.json
+
+# OS cruft
 .DS_Store