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/teach/MISSION-FORMAT.md

@@ -0,0 +1,31 @@
+# MISSION.md Format
+
+`MISSION.md` lives at the workspace root. It captures the _reason_ the user is learning this topic. Every teaching decision — what to teach next, which resources to surface, which exercises to design — should trace back to this document.
+
+## Template
+
+```md
+# Mission: {Topic}
+
+## Why
+{1-3 sentences. The concrete real-world goal the user is chasing. What changes in their life or work when they have this skill? Avoid abstract framings like "to understand X" — push for the underlying outcome.}
+
+## Success looks like
+- {A specific, observable thing the user will be able to do}
+- {Another specific thing}
+- {…}
+
+## Constraints
+- {Time, budget, prior commitments, learning preferences, anything that bounds the approach}
+
+## Out of scope
+- {Adjacent topics the user explicitly does not want to chase right now — protects the zone of proximal development}
+```
+
+## Rules
+
+- **One mission per workspace.** If the user wants to learn two unrelated things, that is two workspaces.
+- **Concrete over abstract.** "Run a half marathon by October" beats "get fitter." "Ship a Rust CLI to my team" beats "learn Rust."
+- **Push back on vagueness.** If the user cannot articulate why, interview them before writing anything. A bad mission is worse than no mission.
+- **Revise when reality shifts.** Missions change. When the user's goal moves, update this file — don't leave a stale mission steering future sessions.
+- **Keep it short.** If `MISSION.md` runs past a screen, it has stopped being a compass and started being a plan.