feat(skills): support isolated bootstrap directories

[jsoncm]

Summary

• add AgentBootstrap::isolated_skill_dirs for hosts that must limit Skill discovery to explicitly supplied roots
• preserve bundled-Skill availability and the existing default discovery behavior
• cover workspace exclusion with a bootstrap-level test

Verification

• cargo fmt --all --check
• cargo test -p aion-agent isolated_skill_dirs_excludes_workspace_skills --lib
• cargo clippy -p aion-agent --tests -- -D warnings

All checks above were run with the repository-pinned Rust 1.96.1 toolchain.

[jiahe0510]

Reviewed cfac6be against the current main. This is safe to merge.

• isolated_skill_dirs is opt-in, so the existing default Skill discovery behavior remains unchanged.
• The implementation correctly delegates to the established bare-mode loader, limiting filesystem discovery to the explicitly supplied project roots while retaining bundled Skills.
• The bootstrap regression test verifies both sides of the boundary: the isolated root is included and workspace Skills are excluded.
• Existing aion-skills coverage already verifies that bundled Skills remain available in bare mode.

Validation completed on the exact merge tree:

• cargo fmt --all -- --check
• cargo clippy -p aion-agent --tests -- -D warnings
• cargo test -p aion-agent
• AionCore aionui-ai-agent compatibility check against this revision

No AionUI behavior changes until a host explicitly opts into the new builder method.

Approved.