Delegation intent: unify reasoning persistence with block lifecycle

Intent:

Implement the unify-reasoning-block-lifecycle ticket by removing the separate ReasoningItem side channel and making the reasoning/thinking block lifecycle the single authoritative path for live streaming and persistence.

Requirements:

Start by mapping the current reasoning paths in llm-worker: Anthropic thinking stream, OpenAI Responses reasoning items, Timeline block handlers, ReasoningItemCollector, Worker finalization, and request serialization round-trip tests.
Remove the separate finalized reasoning event path:
- ReasoningItemEvent;
- ReasoningItemKind;
- ReasoningItemCollector;
- Timeline::on_reasoning_item / dispatch_reasoning_item / reasoning_item_handlers.
Extend the reasoning/thinking block lifecycle so stop/finalization carries all provider material needed to build Item::Reasoning:
- text;
- reasoning item id;
- summary;
- encrypted content;
- Anthropic thinking signature;
- redacted thinking payload metadata.
Convert Anthropic thinking_delta / signature_delta / redacted thinking handling to finalize through reasoning block stop metadata, without emitting a separate ReasoningItem event.
Convert OpenAI Responses completed reasoning items into reasoning block lifecycle events, including metadata-only reasoning where there is no streamed text delta.
Update Worker collection/finalization so Item::Reasoning is built from reasoning block lifecycle state.
Preserve live streaming thinking/reasoning callbacks for UI/trace consumers.
Preserve persisted reasoning history round-trip behavior for Anthropic and OpenAI Responses.
Remove misleading comments that treat reasoning as meta/single-event content.
Do not add backward compatibility aliases or keep duplicate old/new reasoning concepts.

Invariants:

Do not drop provider material required for reasoning round-trip (signature, redacted thinking metadata, id, summary, encrypted_content).
Do not hide model-affecting reasoning persistence in a context-only path; persisted reasoning must remain explainable through committed history items.
Do not redesign unrelated provider request serialization.
Do not read ignored secret-like file contents.
Do not edit the parent workspace; work only in the delegated worktree.
Do not close the ticket, merge the branch, delete worktrees, or push.

Non-goals:

Escalate if:

A provider requires reasoning persistence material that cannot naturally fit in block lifecycle metadata.
OpenAI Responses reasoning items cannot be represented as synthetic block lifecycle events without losing ordering or identity semantics.
The change requires broad public API churn outside llm-worker and its direct consumers.
Existing tests imply a behavior conflict between live streaming callbacks and persistence.

Validation:

Run focused reasoning/timeline/provider tests that cover Anthropic thinking signature/redacted material and OpenAI Responses id / summary / encrypted_content round-trip.
Run cargo test -p llm-worker --lib.
Run cargo check --workspace --all-targets.
Run ./tickets.sh doctor and git diff --check.
Run nix build .#yoi if feasible; record if skipped and why.
Commit the implementation in the worktree when reviewable.

Completion report: