✨ [P2] Better skills loading mechanism

Feature Request

Make adding skills to agents feel natural and Rubyesque. Currently we pass skills in the constructor or call add_skill(). We should support more idiomatic patterns.

Proposed Syntax Options

Option 1: Array-like << and +=

agent = Antigravity::Agent.new
agent.skills << "https://github.com/gemini-cli-extensions/sre"
agent.skills += ["./skills/code-review", "./skills/security-audit"]

Option 2: Block DSL (tap-style)

agent.skills do |s|
  s << "https://github.com/gemini-cli-extensions/sre"
  s << "./skills/code-review"
end

Option 3: Chainable / Builder pattern

agent
  .with_skill("https://github.com/gemini-cli-extensions/sre")
  .with_skill("./skills/code-review")
  .connect!

Option 4: Varargs (Python p() stacking equivalent)

agent.add_skills(
  "https://github.com/gemini-cli-extensions/sre",
  "./skills/code-review",
  "./skills/security-audit"
)

Option 5: Constructor DSL (RubyLLM-inspired)

Antigravity::Agent.new do |a|
  a.model = "gemini-2.5-pro"
  a.skills << "https://github.com/gemini-cli-extensions/sre"
  a.skills << "./skills/code-review"
  a.workspace = "."
end

Option 6: Method missing magic (Rails-style)

agent.skill "https://github.com/gemini-cli-extensions/sre"
agent.skill "./skills/code-review"
# singular = add one, plural = list all
agent.skills  # => [Skill<sre>, Skill<code-review>]

Discussion

The most idiomatic Ruby approach is probably a combination of Options 1 + 5:

• agent.skills returns a SkillSet (custom Array-like class with <<, +=, each)
• Constructor accepts a block for initial config
• SkillSet#<< auto-resolves URLs (GitHub clone) and local paths

This mirrors how Rails does config.middleware, Bundler does gem, and RSpec does config.include.

Current API (for reference)

# Constructor
agent = Antigravity::Agent.new(skills: ["./path", "https://github.com/..."])

# Post-construction
agent.add_skill("./path")
agent.add_skill("https://github.com/...")
agent.add_skills(["./a", "./b"])
agent.add_inline_skill(name: "x", description: "y", instructions: "z")

Priority

P4 -- nice-to-have DX improvement. Current API works fine.

Labels

enhancement, ruby-dx, P4

Start by reviewing the current Antigravity::Agent.new API and the add_skill, add_skills, and add_inline_skill entry points described in the issue. The proposal lists six alternatives without selecting one, so first confirm the intended syntax and SkillSet behavior. Done means the chosen API is implemented while preserving the current API, with tests for the agreed usage patterns.