Fix OpenCode leaked think normalization

[danshapiro]

Preserve leaked OpenCode/Kimi think content as thinking transcript items instead of stripping it from assistant text. Handle balanced tags, malformed leading closer markers, marker-only parts, unterminated open tags, and role-missing OpenCode messages while preserving user-authored text.

Changes:

• Replaces stripThinkTags with itemsFromAssistantTextPart that segments think-tag content into visible thinking items
• Adds NormalizedTextSegment type, normalizeBalancedThinkTags, segmentsToItems helpers
• Changes itemFromPart to return FreshAgentTranscriptItem[] (flatMap) and accept followedByTool param
• Adds computeToolAfterByPartIndex and textSummaryFromItems helpers
• 4 new tests covering balanced, malformed, unterminated, and dangling think tags

Rebase notes: Resolved conflict in normalizeOpencodeTurn — kept main's role-less message rejection policy (null return) while applying the think-tag normalization improvements. Updated one branch test to match main's role-less rejection behavior.

Kata: 0yza