Add git:<url> as a third install source

[tianyilt]

Summary

Adds git:<url> as a third install source for /skill-git:install, alongside the existing skillhub: and clawhub: registries. Lets users install skills from any git URL — including private repos on GitHub/GitLab/self-hosted, beyond what the two public registries cover.

Motivation

/skill-git:install currently only accepts SkillHub or ClawHub identifiers. Both registries serve public skills. Users who:

• maintain skills in private GitHub/GitLab repos (org-internal tools, in-progress work, sensitive content)
• self-host git on company infrastructure
• want to install a fork of a public skill from their own URL

…have no first-class path. The vim-plug / npm / pip ecosystem has long settled on "any git URL" as the universal source — this PR brings the same convenience to skill-git, completing the pattern alongside the curated public registries.

Design

/skill-git:install git:<owner>/<repo>[@<install_name>]      # GitHub shorthand
/skill-git:install git:<full-url>[@<install_name>]          # any host / scheme

• <owner>/<repo> shorthand expands to https://github.com/<owner>/<repo>.git. HTTPS works for public repos with no setup, and for private repos when the user has run gh auth login (gh registers a git credential helper). Users who prefer SSH can pass the full URL: git:git@github.com:owner/repo.git.
• Full URLs are passed to git clone as-is (https://, http://, git@host:, ssh://).
• Optional trailing @<install_name> overrides the auto-derived name (basename of the repo with .git stripped). Useful when the repo is named skill-foo but the user wants to install it as foo.
• Cloned .git and tags are preserved (not stripped). sg-init.sh already handles 4 cases — including "repo with existing v* tags" (Case D, skip) and "repo with commits but no tags" (Case C, tag v1.0.0 on the first commit) — so the existing init step composes cleanly with cloned content.
• Origin remote is left intact, so git pull origin works for upstream updates afterward.

Implementation

File	Change
commands/install.md	Frontmatter description/argument-hint updated. Step 0 adds git:<spec> parsing + install_name derivation. Step 3 adds new sub-section 3c. Git URL; existing "Validate" renumbered to 3d. Preview Source line includes Git — <url>.
scripts/sg-install-git.sh	New helper. Normalizes shorthand to HTTPS, refuses non-empty install paths, runs git clone, validates SKILL.md presence at repo root.

Total diff: +88 / -6 lines, 1 new file.

Tests

Manual reproduction (scripts/sg-install-git.sh directly, since the repo has no test harness):

1. Full file:// URL — clones, preserves .git and v* tags ✓
2. Refuses non-empty target — exits with clear error ✓
3. Missing SKILL.md — exits with clear error pointing to root ✓
4. Shorthand expansion — tianyilt/skill-paper-write → https://github.com/tianyilt/skill-paper-write.git ✓
5. End-to-end against a real private GitHub repo (HTTPS via gh credential helper) — clone succeeds, SKILL.md present, v1.0.0 tag pulled, origin set ✓

Backward compatibility

SkillHub and ClawHub paths are completely untouched. Existing identifiers (skillhub:owner/repo@skill, clawhub:slug) continue to dispatch to Steps 3a / 3b unchanged.

Out of scope (potential follow-ups)

• /skill-git:search git:<query> — searching arbitrary git hosts is non-trivial and not symmetric with the curated registries; left for a separate discussion.
• A --branch / --ref flag for installing a specific branch or tag — easy to add later if there's demand.
• HTTP basic-auth handling beyond what the user's git credential helper provides — out of scope for this change.

[tianyilt]

思路就是类比npm pip install这样 支持对着github装库 因为有些skill还在开发或者私仓阶段 就和yml里面git+一样