feat(cli): add one-command installation for bundled skills

[bestony]

Problem

The package ships three agent skills, but users must leave the context-tree CLI and run separate npx skills add commands to install them:

npx skills add first-tree-ai/context-tree --list
npx skills add first-tree-ai/context-tree --skill context-tree-read

This adds avoidable friction during initial onboarding. A user who already has the CLI should be able to install the recommended Context Tree skills with one command.

Proposal

Add an explicit CLI command such as:

context-tree skills install

The implementation may delegate to the Agent Skills CLI, for example by spawning npx skills add first-tree-ai/context-tree with the required non-interactive arguments. It does not need to reimplement skill discovery or installation.

Suggested behavior:

• Install the recommended bundled set (context-tree-init, context-tree-read, and context-tree-write) by default.
• Complete without interactive skill selection so the command works in scripts and agent workflows.
• Optionally accept one or more --skill <name> arguments for selective installation.
• Forward child-process output and return a non-zero exit code when installation fails.
• Print a clear summary of the installed skills and destination.
• Avoid shell command construction; spawn the executable with an argument array.

Acceptance criteria

• A user can install the recommended bundled skills with one context-tree command.
• The command has deterministic, non-interactive behavior.
• Invalid skill names and installer failures produce actionable errors and a non-zero exit code.
• Tests cover argument construction, successful installation, and installer failure without performing a real global installation.
• --help and the README document the one-command onboarding path.
• Documentation clarifies that this command is an explicit networked operation, since the README currently states that the core and CLI never make network requests.

Example user flow

npm install --global @first-tree-ai/context-tree
context-tree skills install

After the package is available, the second line should be the only command needed to land the bundled skills in the supported agent environment.

[ggordonhall]

This is being reworked in #6. context-tree will be distributed as an Agent Plugin rather than as a Skill: https://agent-plugins.org/. The plugin bundles the npm cli package.

Users can install the plugin by running:

# Codex
  codex plugin marketplace add first-tree-ai/context-tree
  codex plugin add context-tree@context-tree

  # Claude Code
  claude plugin marketplace add first-tree-ai/context-tree
  claude plugin install context-tree@context-tree

I'll see if it's possible to reduce this from two commands to one.

[ggordonhall]

Merged in #11

Command is context-tree install