feat(agents): add .DCP shorthand command and dcp skill

.DCP = Document, Commit, Push — updates in-repo docs and .labyricorn
devlog records for recent milestones, validates, commits, and pushes
to origin/main. Skill defined at .agents/skills/dcp/SKILL.md.
Command documented in root AGENTS.md shorthand commands section.

AGENTS.md

@@ -45,4 +45,13 @@ scoped `AGENTS.md` files before making changes.
 - Alternatively, launch the graphical editor with `python devlog_editor.py` for
   interactive management of devlog entries and publishing status.
 - Run `git diff --check` and review diffs carefully before staging or committing.
-- At handoff, summarize files changed, validation performed, and any next steps.
+- At handoff, summarize files changed, validation performed, and any next steps.
+
+## Shorthand commands
+
+- `.DCP` — **Document, Commit, Push.** When the user issues `.DCP`, read and
+  execute the `.agents/skills/dcp/SKILL.md` skill in full. This means:
+  update any stale in-repo documentation (README, CHANGELOG, docs/), write or
+  update `.labyricorn/` devlog entries for recent milestones, validate with
+  `python devlog_editor.py --validate`, commit the documentation and devlog
+  changes, and push to `origin/main`.