feat: implement config validate/show and dataset validate/describe

[Dongbumlee]

Summary

Implements 4 validation/inspection commands (Tier 2), building on PR #59:

• agentops config validate — full chain validation: run.yaml → bundle → dataset → JSONL
• agentops config show — display resolved configuration with merged paths
• agentops dataset validate — validate dataset YAML + JSONL row-by-row
• agentops dataset describe — show dataset stats (name, fields, rows, metadata)

Architecture

• services/_workspace.py — shared workspace resolution (extracted from browse.py)
• services/validate.py — validation + inspection service layer
• Follows the same pattern as browse.py: dataclass results, thin CLI handlers

Key Features

• config validate checks: YAML syntax, Pydantic schema, file references resolve, threshold evaluator names match evaluator names, JSONL rows have required fields
• config show resolves: paths, backend config, evaluator list, data row counts
• dataset validate checks: dataset YAML schema, JSONL exists, each row is valid JSON with required fields
• dataset describe shows: name, format, input/expected/context fields, row count, all unique fields across rows

Testing

• 19 new unit tests (service + CLI) — all pass
• 115 total tests pass
• Smoke-tested against live workspace

Depends On

This PR targets feature/browse-commands (PR #59). Merge #59 first, then rebase this onto develop.