DX-121772: Add saved Skill CLI commands

[fedexultra]

Summary

Adds a saved Skill command group to dremio/cli so Skills can be listed, inspected, created, updated, and deleted through the existing project-scoped AI Agent REST path.

What Changed

• Added dremio skill list|get|create|update|delete commands using /v1/projects/{pid}/agent/skills.
• Added DremioClient Skill REST methods through the existing HTTP layer.
• Added create/update prompt text support from either inline text or a prompt file.
• Added update merge behavior that fetches the existing Skill, preserves omitted fields, and uses the fetched tag unless --tag is provided.
• Added optional Skill metadata inputs for status, activationScope, whenToUse, comma-separated tags, and repeatable metadata key/value pairs.
• Registered Skill command introspection schemas and README examples.
• Added mocked command/client tests for Skill routing, request bodies, update merge behavior, prompt validation, delete, and API error conversion.

Validation

• uv run ruff format src tests
• uv run ruff check src tests
• uv run pytest tests/ -v (179 passed)
• uv run dremio skill --help
• uv run dremio skill create --help
• uv run dremio skill update --help
• uv run dremio describe skill.create

Notes

• Live API preflight was not run locally because DREMIO_URI, DREMIO_PROJECT_ID, and DREMIO_TOKEN were not set in the environment.
• The REST path and request shapes were checked against the current saved Skill OpenAPI contract in dremio/dremio.

[CLAassistant]

All committers have signed the CLA.