fix(agent-runtime): release an unclosed think-tag open as text at flush

[nordicnode]

Summary

• an explicit <think> open in the content stream no longer commits the rest of the step to the thinking box unconditionally: the hold is released as text at flush() when no close ever arrives
• give the explicit open the same hold machinery the implicit head already uses (holdingForOpen): a close settles the held content as reasoning, a native reasoning chunk disarms it, and a step that ends without a close releases it as text
• the IMPLICIT_OPEN_BUDGET_CHARS give-up stays implicit-head-only — a genuine think block can legitimately run long, and only its close (or the end of the step) settles it
• add regression tests for the prose-quoted open (Write <think> like this in your docs), the hold-until-close streaming shape, the native-reasoning disarm, and the flush release

This addresses the answer-swallowing part of #1155 (the unclosed-think-tag bug): whenever a model emits the tag as prose — writing docs, quoting a template, or a lane whose chat template is broken — the visible answer landed in the thinking box and the user saw an empty or short reply. History still stores the raw text, so this is a display/reclassification fix, not context loss.

Validation

• bun test packages/agent-runtime/src/util/__tests__/think-tag-stream.test.ts
• bun test packages/agent-runtime/src/__tests__/stream-parser-think-leak.test.ts
• bun test packages/agent-runtime (585 pass; the 2 prompts-schema-handling failures and 2 agents-graveyard module-not-found errors pre-exist on main — verified by stashing this change)

Refs #1155

[victorxheng]

Apologies — this PR was auto-closed by GitHub when we force-pushed a history rewrite of this repository (repository maintenance; every commit SHA changed). That was not a judgment on this PR, and GitHub does not allow us to reopen it because the commits it was based on no longer exist in the new history.

If you'd like to continue with this change: rebase your branch onto the new main (or recreate it from a fresh clone) and open a new PR — feel free to link back to this one for context, and we'll pick up the review there.

Sorry for the churn, and thanks for contributing.