Replace hardcoded addon enum with dynamic discovery

[renardeinside]

Summary

• Replace hardcoded Addon enum in apply.rs with dynamic validation against addon.toml manifests — new addons no longer require code changes
• Remove stale SKILL.md.jinja2 from base template (skills install via addon skill_path)
• Replace "stateful apps only" terminology with "requires lakebase addon" across all assistant templates
• Default claude addon to selected during apx init
• Add addon CLI documentation to SKILL.md
• Fix integration test to use current --addons flag

Test plan

• cargo build passes
• cargo test — all 232 tests pass
• cargo run -- dev apply --help — shows dynamic addon list
• cargo run -- dev apply nonexistent — error lists valid addons
• cargo run -- dev apply cursor — works as before
• grep -r "stateful apps only" — zero results
• Fresh apx init defaults to ui + claude addons

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