feat: add skill install service

[pixincreate]

Summary

• add capsync install <reference> as a new service for installing a single skill into the configured skills_source
• support explicit v1 refs (skills.sh skill URLs, GitHub tree URLs, and owner/repo/... skill refs) with temp-checkout-based installation and optional sync
• document the clone vs install mental model and expand regression coverage for install parsing and guardrails

What changed

• added a new install service module and wired it into the CLI
• added capsync install <reference> [--no-sync]
• install now:
  • resolves an explicit skill reference
  • clones the source repo into a temporary directory
  • finds exactly one skill directory with SKILL.md
  • copies that skill into skills_source/<slug>
  • prompts before replacing an existing installed skill
  • refuses to install into a skills_source that is itself a git repo managed by capsync clone
  • runs capsync sync automatically unless --no-sync is passed
• updated README and internal docs to clarify:
  • clone = whole repo checkout in skills_source
  • install = one skill copied into skills_source/<slug>
  • current v1 limitations and supported reference formats

Supported install refs (v1)

• https://skills.sh/owner/repo/skill-slug
• http://skills.sh/owner/repo/skill-slug
• https://github.com/owner/repo/tree/<branch>/path/to/skill
• owner/repo/skill-slug
• owner/repo/path/to/skill

Testing

• cargo fmt
• cargo clippy
• cargo test

[Copilot]

Pull request overview

Adds a new capsync install <reference> workflow for materializing a single skill into the configured skills_source/<slug>, complementing the existing repo-wide clone behavior.

Changes:

• Introduces a new install service that resolves explicit skill references, clones to a temp checkout, selects a skill directory, and copies it into skills_source/<slug>.
• Wires the install command into the CLI with optional post-install sync via --no-sync.
• Expands documentation and adds regression tests around reference parsing and install guardrails.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
tests/install_test.rs	Adds parsing + checkout-install regression tests and guardrail coverage.
src/main.rs	Registers the new install module in the binary crate.
src/lib.rs	Exposes the new install module from the library crate.
src/install.rs	Implements install reference resolution, checkout scanning, and filesystem copy/install behavior.
src/cli.rs	Adds capsync install <reference> [--no-sync] command wiring and output.
README.md	Documents the clone vs install mental model and install usage.
documentation/how-it-works.md	Updates architecture + workflows to include install behavior.
documentation/agents.md	Updates command reference and troubleshooting to include install.

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