feat(tui): add response tokens/sec, paste expand command, and context usage dialog

[SakshamKapoor2911]

Closes#6096, closes#8501, closes#6152

Type of change

• New feature

What does this PR do?

Three TUI improvements:

1. Tokens/sec in response footer — calculates total tokens across all categories divided by wall-clock duration, displayed after the duration in assistant message footers.

2. Paste expand command — prompt.toggle_paste_expand expands all [Pasted ~X lines] placeholders inline using the existing expandTrackedPastedText helper. Available from command palette.

3. Session context dialog — new dialog accessible via /context showing context window bar, token breakdown (input/output/reasoning/cache), cost, and per-message usage.

How did you verify your code works?

• Typecheck passes with no new errors (only pre-existing errors in llm/schema/plugin packages)
• All changes scoped to packages/tui/ — no server/protocol/SDK changes needed
• Uses existing APIs: expandTrackedPastedText for paste expansion, sync store data for context dialog

Screenshots / recordings

UI changes — no screenshots yet, pending local verification.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found related PRs that address the same issues being fixed by PR #37029:

Related PRs:

1. PR feat(tui): add tokens per second to response footer #12721 - "feat(tui): add tokens per second to response footer"

   • Directly addresses issue feat(tui): add tokens per second to response footer #12721 (tokens/sec feature)
   • feat(tui): add tokens per second to response footer #12721

2. PR fix(tui): expand pasted text placeholders literally #33700 - "fix(tui): expand pasted text placeholders literally"

   • Related to the paste expand command feature (issue [FEATURE]: Allow to expand the pasted text (e.g. [Pasted ~1 lines]) #8501)
   • fix(tui): expand pasted text placeholders literally #33700

3. PR feat(tui): show estimated live token throughput in footer #30164 - "feat(tui): show estimated live token throughput in footer"

   • Related to the tokens per second footer feature
   • feat(tui): show estimated live token throughput in footer #30164

These PRs appear to be addressing the same or overlapping features that are being consolidated in PR #37029. You may want to check if these are closed/merged or if PR #37029 supersedes them.

[github-actions]

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