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