feat(agent): add cell-explorer-agent library and CLI

[hweej]

Summary

• New cell-explorer-agent uv workspace package: dataset-scoped LLM chat agent over a provider-pluggable LLMClient protocol (Anthropic adapter ships v1; direct / Bedrock / Vertex transports).
• 7 data tools (get_dataset_schema, describe_obs_column, cluster_stats, search_genes, gene_expression_summary, top_expressed_genes, compare_groups) + 7 UI-action tools that emit partial AppConfig payloads consumed by the frontend viewer.
• Typer CLI cell-explorer-chat single-shot entry point; FakeLLMClient + FakeZarrAccess drive 84 unit/integration tests with no network dependency.
• All tool responses bounded via cap_json_bytes; agent loop enforces max_tool_calls_per_turn, converts tool exceptions into structured tool-results with correlation IDs, and propagates cancellation.

Scope

This is plan 1 of 3 for the chat feature. The FastAPI /api/chat route and the frontend chat package ship in follow-up PRs.

Test plan

• uv run pytest packages/cell-explorer-agent/tests/ -v — 84 passed, 1 skipped (TS drift check only runs in non-worktree checkout)
• uv run cell-explorer-chat --help renders expected usage
• Full workspace suite still green: uv run pytest — 181 pre-existing tests remain passing alongside the new 84
• Manual smoke test against a real dataset with ANTHROPIC_API_KEY set