feat(tui): add thread commands and session graph

[mrbickle98]

Problem

Users need a way to explore specific parts of a conversation in isolation without losing context. Currently there is no way to fork a discussion thread from selected text or visualize the relationship between related sessions.

Proposed Solution

Add two new slash commands:

1. /thread - Creates a child thread from selected text

   • Captures selected text and parent conversation context
   • Shows ThreadFooter with navigation (parent/prev/next) and selected text excerpt
   • Injects parent context as synthetic system reminder on first prompt

2. /graph - Shows session hierarchy tree

   • Displays threads and subagents in a tree view
   • Shows selected text excerpts for each thread
   • Click-to-navigate to any session

Verification

• Typecheck passes: bun typecheck in packages/tui
• Unit tests pass: bun test test/util/session.test.ts

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The title uses conventional commit format (feat(tui):) instead of the required Feature Request title format, which should start with [FEATURE]:
• The issue is missing the Feature Request template structure, specifically the verification checkbox confirming the feature hasn't been suggested before

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

---

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: /tree command for visual session navigation #22067: Proposes a /tree command for visual session navigation with click-to-navigate between branches — closely overlaps with the proposed /graph command
• [FEATURE]: Multiple chat tabs per session in web/desktop #8550: Proposes multiple chat tabs per session for parallel exploration, overlapping with the thread forking concept

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

This issue has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new issue that follows our issue templates.