feat(claude-skill): add Claude Code skill installer and brev-cli skill

[theFong]

Summary

• Add brev claude-skill command with install / uninstall subcommands
• Integrate skill installation prompt into brev login flow (detects if Claude Code is installed)
• Add standalone bash installer script for non-brev users
• Add comprehensive brev-cli skill with prompts, reference docs, and examples
• Support BREV_SKILL_BRANCH env var for testing from branches
• Document Claude Code integration in README

Usage

# Install the skill
brev claude-skill install

# Uninstall the skill
brev claude-skill uninstall

# Default (install)
brev claude-skill

# Test from a branch
BREV_SKILL_BRANCH=pr/claude-skill brev claude-skill install

Test plan

• brev claude-skill install downloads all skill files
• brev claude-skill uninstall removes the skill directory
• brev claude-skill defaults to install
• BREV_SKILL_BRANCH=pr/claude-skill brev claude-skill install pulls from branch
• brev login prompts skill installation when Claude Code is detected
• Standalone installer: curl -fsSL .../install-claude-skill.sh | bash

[theFong]

Manual QA Results ⚠️

Build & Static Analysis:

• ✅ Build passes
• ✅ go vet passes

Test Plan Verification:

• ⚠️ brev claude-skill install - Cannot test pre-merge: Downloads from GitHub main branch where files don't exist yet
• ✅ brev claude-skill --uninstall removes skill - Verified: Successfully removes skill directory
• ⚠️ brev login skill prompt - Needs integration testing
• ⚠️ Standalone installer script - Cannot test until files are on main

Skill Files Verification:

• ✅ SKILL.md is well-structured with proper YAML frontmatter
• ✅ All referenced prompts/ and reference/ files exist
• ✅ Safety rules and common workflows documented

Can use the branch name env var for testing

Post-Merge Testing Required:

• Full install flow
• Login integration prompt
• Standalone bash installer

[theFong]

https://docs.google.com/document/d/1pYYR1nfcfJi9kCtcxiY5DINYjGxXwnO6zh7JdtmGBTU/edit?tab=t.0#heading=h.oyak6y4d25ei

[theFong]