[2/5] Aitools: install writes state, interactive agent selection, idempotent install

[simonfaltum]

PR Stack

1. [1/5] State + release discovery + directory rename ([1/5] Aitools: state tracking, manifest source, directory rename #4810)
2. [2/5] Install writes state + interactive agent selection (this PR)
3. [3/5] Update + uninstall + version commands ([3/5] Aitools: update, uninstall, and version commands #4812)
4. [4/5] List improvements + command restructuring + flags ([4/5] Aitools: list command, flat structure, --skills/--agents/--include-experimental flags #4813)
5. [5/5] Project scope (--project/--global) (pending)

Base: simonfaltum/aitools-pr1-state (PR 1)

Why

Install currently has no state tracking, no filtering, and outputs every file download to the console. Users can't tell what version they have, and there's no way to install for specific agents.

Changes

Before: skills install downloads everything, prints every file, has no state, no filtering, no agent selection.

Now:

• InstallSkillsForAgents(ctx, src, agents, opts) as the core install function. InstallAllSkills becomes a thin wrapper (signature preserved for cmd/apps/init.go compat).
• State written to .state.json after successful install. Merges with existing state (doesn't overwrite).
• Idempotent: second install skips already-present skills with matching versions.
• Experimental filtering: skip experimental: true skills by default.
• min_cli_version enforcement: skip incompatible skills with warning (hard error for single-skill install).
• Interactive agent selection via charmbracelet/huh multi-select. Skip prompt for 1 agent, all detected for non-interactive.
• Legacy install detection: prints "reinstall" message when old install found without state.
• Concise default output (2 lines). Per-file detail only at debug level.

Test plan

• State created after install-all and install-single
• Experimental filtering (skip by default, include with flag)
• min_cli_version: warning for all, hard error for single
• Idempotent: skip same version, update changed
• Legacy detection with helpful message
• Interactive prompt for 2+ agents, skip for 1
• Non-interactive: all detected, no prompt
• InstallAllSkills signature preserved
• Concise output, per-file at debug level

[github-actions]

Suggested reviewers

Based on git history of the changed files, these people are best suited to review:

• @arsenyinfo -- recent work in experimental/aitools/lib/installer/, experimental/aitools/cmd/

Confidence: high

Eligible reviewers

Based on CODEOWNERS, these people or teams could also review:

@databricks/eng-apps-devex, @lennartkats-db

_{Suggestions based on git history of 5 changed files (4 scored). See CODEOWNERS for path-specific ownership rules.}