[codex] add python workspace run tracking

[ashione]

What changed

This PR adds a Python-side workspace layer for tracked local runs and artifact indexing.

Key changes:

• add python_api/velaria/workspace/ with paths, run_store, artifact_index, and types
• extend python_api/velaria_cli.py with:
  • run start/show/status/cleanup
  • artifacts list/preview
  • tracked adapters for csv-sql, vector-search, and stream-sql-once
• add Python binding support for DataFrame.explain() and StreamingQuery.snapshot_json()
• add a Bazel sync target and helper script to copy the built native extension back into the source checkout
• update regression scripts, docs, AGENTS guidance, and the user skill
• add tests for workspace run persistence, artifact indexing, CLI contract failures, and stream progress persistence

Why it changed

The repo needed a minimal local workspace model for agent/skill invocation:

• every execution should have a stable run_id
• run metadata, explain, progress, logs, and artifacts should land in a local run directory
• artifacts should be queryable through a lightweight local index
• CLI failure paths must stay machine-readable for automation

Impact

User-facing behavior:

• tracked runs now persist under VELARIA_HOME or ~/.velaria
• artifacts can be listed and previewed through the CLI
• streaming runs now persist native progress snapshots line by line
• cleanup now skips active runs by default
• CLI error paths return JSON instead of Python tracebacks on stdout

Implementation constraints preserved:

• kernel checkpoint semantics are unchanged
• SQLite is only used for metadata and small previews, not as a result engine
• workspace persistence keeps the native explain/progress contract unchanged

Root cause

Previously the Python ecosystem layer did not provide a persistent workspace abstraction for local execution history, and the CLI only supported immediate command output. That made agent-oriented local traceability, artifact discovery, and machine-readable status inspection incomplete.

Validation

Passed:

• uv run --project python_api python -m unittest python_api.tests.test_artifact_index
• uv run --project python_api python -m unittest python_api.tests.test_python_cli_contract
• uv run --project python_api python -m unittest python_api.tests.test_workspace_runs
• uv run --project python_api python -m unittest python_api.tests.test_streaming_v05
• ./scripts/build_py_cli_executable.sh /tmp/velaria-cli-fix-dist velaria-cli-fix
• previously in this branch flow: core/python/experimental/stream observability regression scripts passed before the final docs-only updates