t3-provision-users: vendor agent skills + per-user install_skills (emo)

Make the admin's Claude Code agent skills available to the `emo` devvm user.
Viktor asked to install Matt Pocock's skills for emo, starting with grill-me
but covering the full set the admin already uses.

The `npx skills` upstream has drifted off that set (diagnose -> diagnosing-bugs
and write-a-skill -> writing-great-skills were renamed; caveman + zoom-out are
no longer published), so reproducing it via npx is impossible and would also
spray ~70 agent dirs into the user's home + add a GitHub-clone + unpinned-CLI
dependency to the hourly root reconcile. Instead vendor a point-in-time
snapshot of the 16 skills (scripts/workstation/claude-skills/) and copy them
per-user, mirroring install_memory: install_skills() copies each skill into
~/.agents/skills/<name> (owned by the user) and symlinks
~/.claude/skills/<name> -> ../../.agents/skills/<name>. if-absent, additive,
best-effort, scoped to the SKILL_USERS allowlist (emo).

find-skills is from vercel-labs/skills (not Matt Pocock) but included since it
is part of the admin's current set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/workstation/claude-skills/handoff/SKILL.md

@@ -0,0 +1,13 @@
+---
+name: handoff
+description: Compact the current conversation into a handoff document for another agent to pick up.
+argument-hint: "What will the next session be used for?"
+---
+
+Write a handoff document summarising the current conversation so a fresh agent can continue the work. Save it to a path produced by `mktemp -t handoff-XXXXXX.md` (read the file before you write to it).
+
+Suggest the skills to be used, if any, by the next session.
+
+Do not duplicate content already captured in other artifacts (PRDs, plans, ADRs, issues, commits, diffs). Reference them by path or URL instead.
+
+If the user passed arguments, treat them as a description of what the next session will focus on and tailor the doc accordingly.