fixes https://github.com/dyad-sh/dyad/issues/2500

## Summary
- Adds `filterOrphanedReasoningParts()` function to filter out OpenAI
reasoning items that aren't followed by output items
- Fixes error: "Item of type 'reasoning' was provided without its
required following item"
- This error occurs with OpenAI reasoning models (o1, o3, o4-mini) when
conversation history contains orphaned reasoning items

## Test plan
- Added 4 unit tests covering:
  - Reasoning with following output is preserved
  - Orphaned reasoning (alone in message) is filtered out
  - Reasoning followed by tool-call is preserved
  - Trailing reasoning after text output is filtered
- All 672 tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches message-history shaping sent to OpenAI (including multi-step
local-agent flows); incorrect filtering could drop reasoning parts or
change message arrays, potentially impacting model behavior, but changes
are scoped and covered by new unit tests.
> 
> **Overview**
> Prevents OpenAI Responses API errors by **cleaning conversation
history** before reuse: message parts now have stale
`providerOptions/providerMetadata.{openai,azure}.itemId` removed and
**orphaned `reasoning` parts (not followed by any non-reasoning
output)** filtered out via `cleanMessageForOpenAI()`.
> 
> This cleaning is applied both when parsing persisted `aiMessagesJson`
(`parseAiMessagesJson`) and during local-agent multi-step
`prepareStepMessages`, with expanded unit tests covering preservation vs
filtering cases; `thinking_utils` also disables OpenAI `store` for
`local-agent` mode and `AGENTS.md` documents the failure mode and fix.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
5b0c70c2817345598e2eb88dcefbb0f859d6bb88. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Filters orphaned reasoning parts in conversation history to prevent
OpenAI Responses API errors when a reasoning item isn’t followed by
output. Applies filtering when parsing stored messages and during
multi-step prepare step.

- **Bug Fixes**
- Replaced stripItemIds with cleanMessageForOpenAI in
parseAiMessagesJson and prepareStepMessages to strip provider itemId and
filter orphaned reasoning parts.
- Preserves reasoning when followed by text or tool-call, preventing
“reasoning without required following item” errors on o1/o3/o4-mini;
sets store: false to avoid expired item references.
- Expanded tests for parsing and multi-step flows; documented behavior
and fix in AGENTS.md.

<sup>Written for commit 5b0c70c2817345598e2eb88dcefbb0f859d6bb88.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>