## Summary

Promotes the **aitools skills-management surface** out of
`experimental/` so the stable half lives at `databricks aitools …` and
slots in next to the other top-level command groups. The matching
**interface changes** (`--scope` enum, `--project`/`--global`
deprecation, `--agents` auto-detect doc) live in a stacked follow-up:
**#5234**.

This is mostly a move, but it is not move-only — see [Non-move
changes](#non-move-changes) below.

- Source files for `install`, `update`, `uninstall`, `list`, `version`
(and the agents/installer libs they depend on) physically move from
`experimental/aitools/` to `cmd/aitools/` + `libs/aitools/`, matching
the existing `cmd/apps/` + `libs/apps/` layout. OWNERS, Taskfile, and
the pr-checklist skill are updated to match.
- The top-level command is registered at `databricks aitools …`.
- **Keeps the `tools` subtree under `experimental/aitools/`** — `query`,
`discover-schema`, `get-default-warehouse`, `statement …` — because
`tools.go` still says "There are no stability guarantees for these
tools".
- The old paths under `databricks experimental aitools
install/update/uninstall/list/version` and `databricks experimental
aitools skills install/list` keep working as **deprecated
backward-compat aliases** that print a notice pointing at the new path
(via cobra's `Deprecated` field).

The aitools skills-management surface is feature-complete after the 5-PR
series (#4810–#4814) that added state tracking, lifecycle commands, and
project scope support. The `tools` subtree is functionally useful but
its shape is still in flux, so promoting only the stable half.

## Non-move changes

In addition to the file moves, this PR:

- Removes the redundant `aitools/README.md` (apps/pipelines don't carry
one); the same info lives in the command's Long description.
- Rewrites the Long description on the new top-level `databricks
aitools` command.
- Adds a deprecation notice to every legacy alias under `databricks
experimental aitools` (Lennart's review ask — they used to forward
silently).
- Refactors how the legacy `experimental aitools skills install [name]`
wrapper is wired: the wrapper now lives in
`cmd/aitools/legacy_skills.go` alongside the install code it wraps, and
the previously-exported test-injection vars (`InstallSkillsForAgentsFn`,
`ListSkillsFn`, `PromptAgentSelection`) are back to unexported. The two
now-empty experimental files (`experimental/aitools/cmd/skills.go` and
`skills_test.go`) are deleted.

## What's not in this PR

These are deliberately separated and reviewed independently:

- **#5234** — `--scope=project|global|both` flag, deprecation of
`--project`/`--global` via `cobra.Deprecated`, `--agents` auto-detect
help text.
- **#5233** (draft) — `--output json` on `databricks aitools list`.

## Command shape after this PR

```
# Stable, top-level
databricks aitools install      # use --skills <name>[,<name>...] for specific skills
databricks aitools update
databricks aitools uninstall
databricks aitools list
databricks aitools version

# Backward-compat aliases (print deprecation notice; point at the new paths)
databricks experimental aitools install/update/uninstall/list/version
databricks experimental aitools skills {list,install}

# Experimental, unchanged path
databricks experimental aitools tools query
databricks experimental aitools tools discover-schema
databricks experimental aitools tools get-default-warehouse
databricks experimental aitools tools statement {submit,get,status,cancel}
```

## Test plan

- [x] `databricks aitools --help` shows
install/update/uninstall/list/version (no `tools`)
- [x] `databricks --help` lists `aitools` in the output
- [x] `databricks experimental aitools install` prints a deprecation
notice and still forwards
- [x] `databricks experimental aitools tools query …` runs as before
- [x] `databricks experimental aitools tools --help` lists
query/discover-schema/get-default-warehouse/statement
- [x] Existing aitools tests pass; the legacy-wrapper tests moved with
the wrapper to `cmd/aitools/legacy_skills_test.go`

This pull request was AI-assisted by Isaac.

---------

Co-authored-by: simon <simon.faltum@databricks.com>
Co-authored-by: simon <4305831+simonfaltum@users.noreply.github.com>