Make agent run skill flag repeatable

[bnavetta]

Summary

• Make local agent run --skill repeatable while preserving command-line order.
• Resolve every provided skill and register all resolved skills with SkillsManager.
• Keep only the first resolved skill on the invoked-skill path for prompt/task configuration.
• Add product and technical specs plus focused parser and skill-manager tests.

Tests

• cargo nextest run --manifest-path /workspace/warp/Cargo.toml -p warp_cli agent_run
• CARGO_BUILD_JOBS=1 cargo nextest run --manifest-path /workspace/warp/Cargo.toml -p warp handle_skills_added_registers_multiple_explicit_skills
• cargo fmt --all --manifest-path /workspace/warp/Cargo.toml -- --check

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/859faac9-88c6-4cd1-b33e-a37f3efe44fd
Run: https://oz.staging.warp.dev/runs/019e34e0-eae3-7f3a-b66c-ca1e13e3c6f5
This PR was generated with Oz.