feat: add SubAgentTool::with_skills

[yuanhao]

Summary

Adds SubAgentTool::with_skills(SkillSet), mirroring the existing Agent::with_skills. Previously skills could only attach to a top-level Agent, so a skill-using parent dispatching a sub-agent dropped all skill context — the sub-agent couldn't follow skill recipes (it answered from parametric knowledge instead of using the skill's tools/recipe).

The skills index (XML per the AgentSkills standard) is appended to the sub-agent's system prompt at dispatch time, before the shared-state block.

Closes #45

Backward compatibility

Purely additive — existing callers are unaffected:

• New with_skills method; no existing signatures change.
• New struct field defaults to an empty skills prompt (a no-op append).
• SubAgentTool fields are private and the only constructor is new(), so adding a field is non-breaking.

Semver-minor. Downstream apps need no changes; they opt in via .with_skills(set).

Tests

Covers all three branches of the prompt-assembly logic:

• appended to a non-empty base prompt,
• used as the whole prompt when the base is empty (verbatim, no stray newline),
• empty-SkillSet no-op,
• ordering before the shared-state block.

Tests use a shared CapturingProvider (asserts on the provider-observed system prompt) and per-test temp dirs with RAII cleanup to stay safe under parallel execution.

Verified: cargo fmt --check, cargo clippy --all-targets (no warnings), full cargo test.

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[yuanhao]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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