init: offer to run `skilltree teach` when bundled skill missing

[imarios]

Problem

Surfaced during PR #155 (demo refresh). The doctor bundled-skill check warns when the skilltree skill is missing from a detected agent's home dir, with remediation Run \skilltree teach` to install/update the skilltree skill. This is a one-time global setup, but new users discover it only via doctor` — which they may never run.

A returning user has the skill (it persists across projects), so the warning is invisible to them. A first-time user sees it but has no signal to act until they happen to run doctor.

Proposal

When skilltree init detects agents in the user's home, also check whether the bundled skilltree skill is installed for each. If any are missing, offer to run teach inline:

Detected agents: claude, cursor

Enrol detected agents as install targets? [Y/n/1,2] y

The skilltree skill isn't installed for claude, cursor.
Install it now (recommended)? [Y/n] y
…running \`skilltree teach\`…
✔ Installed to 2 agents

--yes should auto-accept; non-interactive runs should print a hint and skip (no surprise writes outside the project dir).

Why this matters

• Closes the gap that made the demo recording surface a confusing warning even after install (fix: adaptive prompt hint + clearer bundled-skill wording + demo/README refresh #155 worked around it by pre-running teach in the demo's hidden setup).
• First-run doctor is then clean by default, which makes the check noisier-but-cheaper to keep.
• The current state effectively requires every user to read the doctor remediation and copy a command.

Out of scope

• Changing doctor's check or its severity — the check stays as-is.
• Auto-installing without a prompt (writes outside project dir without consent).

Found while working on #155.