provider: add uv SandboxProvider (PR #122 stage C)

[Meirtz]

Third slice of the #122 split (stage map in the coordination comment; A = #133, B = #134). Cherry-picks the UvProvider from origin/abridge/gateway-tito (efedacf + the ruff hunk from 90f970a), plus fixes from the pre-PR adversarial review.

What's in

plugins/providers/uv — a lightweight SandboxProvider that skips Docker/Nix entirely: uv venv + uv pip install -e <project> (or reuse_venv=), then the runtime server runs as a local subprocess. For dev / eval / CI where container isolation isn't needed; registered as entry point uv.

Fixes on top of the port

• stdout drain (the planned deadlock fix): nothing read the server's pipe after the health check — asyncio's flow control pauses reading at ~192KiB buffered, so a server that logged past that blocked mid-write and wedged every in-flight rollout. Now drained for the sandbox's lifetime into a bounded 8×64KiB tail, which also feeds the exited-before-health diagnostic.
• CancelledError masking the crash diagnostic: the diagnostic path used wait_for, which cancels the drain task on timeout; delete()'s re-await then surfaced a bare CancelledError instead of the RuntimeError with rc + log tail (reproduced with a grandchild holding the pipe open). Switched to non-cancelling asyncio.wait.
• Port-collision guard: the allocated port is only bound by the subprocess seconds later — concurrent create()s could collide (worst case: two rollouts silently sharing one runtime). Reserved in-process until the sandbox dies, mirroring DockerProvider's _inflight_ports.
• Temp-root leak: a failed uv venv / uv pip install now removes its mkdtemp root (a retry loop leaked one partial venv per attempt).
• Packaged-uv resolution: the default binary resolves via uv.find_uv_bin() — the wheel's binary isn't on PATH under systemd/cron/absolute-path launches, exactly where no system uv exists; a bare FileNotFoundError now carries install guidance.
• README/docstring examples call provider.aclose() (the only thing that removes a materialized venv).

Verification

• uv run pytest tests/ plugins/ — 496 passed (10 uv-provider tests: real subprocess servers via a fake-venv shim — flood/drain, grandchild-held pipe, port guard, temp cleanup, off-PATH resolution, plus the ported remote() round-trip)
• uv run ruff check . / uv run pyright — clean / 0 errors
• Adversarial review: 6 findings (2 major), all fixed above

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