Run each Cloudflare conversation as a single Agents SDK state machine

[astrobot-houston]

Originally proposed by @mattzcarey in #736. Their description is reproduced verbatim below.

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Continues discussion #709 and supersedes #700: the Cloudflare coordinator now runs each conversation as one durable state machine on the Agents SDK engine, instead of a drive run plus one attempt run per submission.

Depends on the unreleased Agents SDK engine in cloudflare/agents#2301 (which is stacked on #2274). Draft until an agents release carries it; locally it was built against that branch through a workspace link: override that is not committed.

What changes

• One run per conversation, flue:conversation@v1, at a fixed address. Its idle phase parks on the run's mailbox — holding no alarm — until admission or an abort sends it a wake, then claims the next runnable head from the submission ledger. Its turn phase processes that submission to settlement and returns to idle, where the next queued head is claimed in the same invocation. The submission ledger stays the record of what was accepted and how it settled (it is shared with the Node target); the machine is its executor.
• Interruption is a replayed turn. A turn re-dispatched without the in-isolate claim memory reconciles the running row from the canonical stream, exactly as the attempt run did — same reconcileInterruptedSubmission, same classification.
• A hung turn is abandoned, not fatal. The SDK's per-transition watchdog runs the machine's onCancel, which aborts and orphans the attempt's controller (so a zombie's appends are refused) and returns the machine to idle, where the ledger settles the submission through the timeout classification. Live turns heartbeat the watchdog until the submission's own timeout, which aborts their work from inside.
• Aborts are honoured in-isolate at once and durably through the mailbox.
• The generated agent class declares the machine on taskDefinitions; the coordinator loses ensureDrive, ensureAttemptRun, hasLiveAttemptRun and the per-attempt run ids (~600 lines net).

Verified

pnpm check:types (109 tasks), the runtime and vite builds, and the runtime unit tests pass. Live on examples/cloudflare against the engine build: a prompt settles completed; two prompts queued back-to-back settle with the second joined at the turn boundary; an abort mid-response settles aborted and a follow-up prompt completes; killing the dev server mid-response and restarting it replays the turn, which continues from the durable partial stream and settles completed. The machine's row parks waiting on its mailbox with no wake job while the conversation is quiet.

Docs: the "Durable agent execution" section of the Cloudflare target guide and "Cloudflare recovery" in the durability guide describe the new model.

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Original implementation from #736 by @mattzcarey