[skills] Record the workspace org id in downloaded-skill attribution

[xsh310]

What

Record the numeric workspace (org) id as workspace_id on each downloaded-skill
attribution record. Final PR of the attribution work, on top of #585.

Why

The record already stores the workspace URL, but a URL can change (workspace
rename / vanity host). The numeric org id is a stable, rename-proof workspace
identifier, so attribution keeps both. It costs nothing extra to capture:
Databricks stamps every authenticated response with an X-Databricks-Org-Id
header, and the download already makes those calls.

Changes

• databricks.py: the HTTP GET helpers capture X-Databricks-Org-Id into a
  session cache keyed by hostname (mirroring the existing model-service listing
  caches); workspace_org_id(workspace) reads it back, and a cache-clear hook is
  wired into the test isolation fixture. No extra request is made.
• skills_state.py / skills_download.py: SkillInstall carries workspace_id,
  set from workspace_org_id when a record is written. metastore_id remains the
  logical key; the field is omitted when no response has revealed the id yet.

Testing

uv run ruff check, ruff format --check, ty check src, and uv run pytest all
pass (the pre-existing test_e2e_user_agent / test_claude_smart_routing_v2
failures also fail on the base). Added coverage that the GET helper captures the
header (and tolerates its absence), and that a download records workspace_id
when the id is known and omits it when it is not.

Stack

Part of gh stack #576, on top of #585 (skills-download-remove).

This pull request and its description were written by Isaac.

[sunishsheth2009]

P2: workspace_id is now recorded, but the install identity key (_record_key → (metastore_id, fqn, base)) doesn't include it, and the SkillInstall docstring still describes the key without it. When metastore_id is None (older/edge records), two skills with the same fqn at the same base from different workspaces collide on reconcile and one overwrites the other. Either fold workspace_id into the identity key or update the docstring to state it's descriptive-only.