refactor(tools): collapse the opaque-tool watchdog to a standard timeout (#1472 step 5)

[Aaronontheweb]

What & why

#1472 step 5 — "addition through subtraction" of the tool-liveness machinery, at the opaque-tool boundary.

After #1481, the off-thread, volatile-polling StreamingToolWatchdog was used for exactly one thing: bounding opaque tool calls with a flat wall-clock budget. In WallClock mode its two shared Volatile fields (lastActivity, budgetTicks) are written once and never change, so the 1-second polling timer only ever computes "has ToolExecutionTimeout elapsed since start?" — which is exactly what a CancellationTokenSource timeout token does. This was the "why is this volatile instead of actor state?" smell.

Change

Opaque tools are now bounded by a TimeProvider-driven timeout token (new CancellationTokenSource(timeout, timeProvider)) linked to the caller token, drained through the same DrainToCompletionAsync helper self-monitoring tools already use:

• No volatile, no polling timer, no hand-rolled budget state.
• Per call — a slow tool times out without affecting its siblings.
• A budget trip converts to the same TimeoutException message; caller (turn/user) cancellation is rethrown unchanged (preserves the refactor(subagents): consolidate sub-agent liveness — guard, delete suspend flag, stop parent over-supervision #1481 cancel-handling).
• TimeProvider-virtualizable, so tests stay deterministic via FakeTimeProvider.

Deletes StreamingToolWatchdog, ToolWatchdogBudget, ToolWatchdogResetMode.

Direction was chosen deliberately over a literal "actor-owned timer": the work runs off the actor thread, so an actor-owned timer would add per-call cancellation-token plumbing across that boundary — more moving parts, against the goal. Collapsing to a standard timeout removes the volatile rather than relocating it.

Folds in three deferred #1481 review follow-ups

• Bump Microsoft.CodeAnalysis.CSharp from 4.12.0 to 5.0.0 #11 — duplicate drain loop: opaque + self-monitoring now share DrainToCompletionAsync.
• Bump Microsoft.Extensions.Configuration.Json from 10.0.2 to 10.0.3 #12 — SpawnAgentStreamingTests drove spawn_agent through the watchdog (a path production no longer takes); now drains via the production path.
• Bump Microsoft.Extensions.Configuration.Json and Microsoft.Extensions.Hosting #13 — the dead ResetOnItem/FirstItem/InterItem machinery is gone with the class.

No behavior change

Opaque tools still time out at the same wall-clock budget, same message, per-tool isolation. Self-monitoring tools untouched. The spec-of-record (Per-call liveness by tool class in the streaming-tool-call-execution change) already describes opaque tools as "one wall-clock budget" — only the mechanism changed, so no spec edit.

Verification

• Full Netclaw.Actors.Tests green (2474). Existing pipeline tests already cover opaque wall-clock timeout (FakeTimeProvider) and concurrent independence; added one for the stream-without-a-completion-item contract. Gutted the watchdog unit tests, keeping the INetclawTool default-adapter test.
• dotnet slopwatch analyze clean; copyright headers present.
• Net −273 LOC (94 insertions / 367 deletions).

Part of #1472.

[Aaronontheweb]

LGTM