run_agents resolves repo-qualified child skill specs

[warp-dev-github-integration]

Summary

run_agents remote children now resolve repository-qualified child skill specs (org/repo:path/to/SKILL.md and repo:skill-name) in the parent client instead of requiring absolute filesystem paths. The existing active-skill lookup remains the fast path for absolute and bundled references.

Implementation

• Reused the existing client resolve_skill_spec resolver as a native-client fallback in the orchestration runtime.
• Threaded the parent working directory through GUI and TUI RemoteChildLaunchConfig callers.
• Base64-encoded resolved ParsedSkill content into the existing runtime_skills payload.
• Included the ResolveSkillError reason in unresolved-skill messages.
• Added regression coverage for org/repo paths, repo-qualified simple names, absolute paths, bundled IDs, and missing repositories.
• No server or proto changes.

Verification

• ./script/format passed.
• ./script/check_no_inline_test_modules passed.
• ./script/presubmit was attempted; its formatting and inline-test stages passed, but clippy could not complete in this sandbox because the repository Cargo target/registry mounts were full. Redirecting Cargo to /tmp progressed further, but the warp rustc process was SIGKILLed by the environment. No source-level compiler diagnostic was emitted.
• Focused regression test compilation was also attempted and hit the same sandbox resource limit.

Originating thread: https://warpdotdev.slack.com/archives/C0BDQDW8V5E/p1784921048509639

Conversation: https://staging.warp.dev/conversation/cc9dde35-bd3d-48d2-af09-7b757324c104
Run: https://oz.staging.warp.dev/runs/019f95f3-0fc4-751e-ac48-5792a54dce14

This PR was generated with Oz.