skills: build MCP and CLI variants from shared templates

[jurassix]

Summary

• Moves skills/ to templates/skills/ as the single source of truth
• Adds a Go build tool (cli/hack/buildskills) that generates two output trees from those templates: skills/ (MCP plugin) and cli/skills/ (CLI binary)
• Template tokens: {{tool "live-connect"}} expands to `live-connect` for MCP or `subtext live connect` for CLI; {{if eq .Target "cli"}} / SKILL.cli.md overrides handle deeper prose divergence
• Committed generated outputs for both targets; CI drift check fails if templates are edited without re-running go generate ./...

Coverage:

• 8 CLI skills: comments, docs, live, proof, recipe-sightmap-setup, shared, sightmap, tunnel
• 14 MCP skills (all of the above + review, session, onboard, first-session, setup-plugin, using-subtext)

Key divergence handled via SKILL.cli.md overrides:

• shared — CLI explains subtext sightmap upload instead of the Python side-band script
• live / tunnel — removes "subtext-tunnel MCP server" references; adapts flow steps to subtext commands
• sightmap / recipe-sightmap-setup — replaces the Python upload script with subtext sightmap upload

Stacks on #75 (the Go CLI port PR).

Test plan

• cd cli && go generate ./... produces no git diff in skills/ or cli/skills/
• cd cli && go test ./... passes (embed smoke test included)
• Spot-check cli/skills/comments/SKILL.md: tool refs show subtext comment list not comment-list
• Spot-check skills/comments/SKILL.md: tool refs show comment-list (byte-identical to pre-move content)
• cli/skills/shared/ has no collect_and_upload_sightmap.py; skills/shared/ does