Add relayed auth MPS support for Claude Max/Enterprise

[asujithan]

Changes

• Relayed refresh proxy (gateway_proxy.py, new) — a loopback proxy that injects a freshly-minted X-Databricks-AI-Gateway-Token on every request while the subscription OAuth stays in Authorization, and streams responses back verbatim. Token is refreshed off the request path.
• Relayed launch path (claude.py, agents/init.py, databricks.py) — auto-detects a credential-less Anthropic MPS (relayed), routes the launch through the proxy, omits apiKeyHelper (so it can't outrank the subscription OAuth), and skips launch-time validation (the proxy/login only exist at launch).
• —setting-sources fix — relayed launches pass —setting-sources project,local, excluding the user scope so a stale ~/.claude/settings.json apiKeyHelper can't merge in and shadow the subscription OAuth. Non-relayed launches are unchanged.
• Method-agnostic proxy dispatch — the proxy forwards any HTTP method (routes any do_ to _handle) instead of a hardcoded subset, so unsupported methods get the gateway's real response instead of a fabricated local 501.
• —workspace launch flag — ucode claude|codex --provider --workspace runs without a prior ucode configure: it targets (and auto-configures + authenticates) the given workspace, reusing the existing auto-configure/auth path.

Test Plan

All via uv run:

• pytest tests/test_gateway_proxy.py — proxy token cache, swap-header injection, streaming, method-agnostic dispatch — pass
• pytest tests/test_agent_claude.py — relayed overlay (no apiKeyHelper), —setting-sources argv, launch wiring — pass
• pytest tests/test_agents_init.py — relayed provider resolution + config threading —pass
• pytest tests/test_databricks.py — relayed detection — pass
• pytest tests/test_cli.py — —workspace sets the current workspace (normalized); absent flag leaves it unchanged — pass
• Combined touched suites: 421 passed. Full suite: 938 passed, 36 skipped (the one failure, test_e2e_user_agent, is a pre-existing environmental e2e test unrelated to this branch).
• ruff check, ruff format --check, ty check — clean.