Add top-level skills install command

[sdairs]

Summary

• add a new top-level \ command for installing bundled ClickHouse agent skills
• support interactive multi-select installs plus non-interactive \ and \ flows
• detect supported agent folders, write the bundled skill package, and document the new command

Testing

• cargo test

[Copilot]

Pull request overview

Adds a new top-level clickhousectl skills command that installs bundled ClickHouse agent skills into supported agent configuration directories, with interactive selection and non-interactive flag-driven flows.

Changes:

• Introduce skills subcommand + CLI args, wiring it into main command dispatch and error handling.
• Implement download + tarball extraction + installation logic for agent skills, including interactive TTY pickers and unit tests.
• Document the new command and ignore common agent config directories in this repo.

Reviewed changes

Copilot reviewed 13 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/skills.rs	Implements skills download/extract/install logic, interactive selection UI, and tests.
src/cli.rs	Adds skills subcommand and SkillsArgs parsing/tests; updates help text.
src/main.rs	Wires the new skills command into top-level dispatch.
src/error.rs	Adds Error::Skills for skills-related user-facing errors.
README.md	Documents clickhousectl skills usage, supported agents, and scope behavior.
Cargo.toml / Cargo.lock	Adds flate2 + tar dependencies for archive handling.
.gitignore	Ignores agent config directories in this repo.
src/version_manager/*, src/server.rs, src/cloud/*	Formatting-only changes (no functional deltas apparent in diff).

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