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 in packages/tui passes with no new errors
• All changes scoped to packages/tui/ — no server/protocol/SDK changes
• 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 the search results, I found one potentially related PR that overlaps with this PR's features:

Related PR:

• feat(tui): add tokens per second to response footer (PR feat(tui): add tokens per second to response footer #12721)
  • This PR already implements the "tokens/sec in response footer" feature that is part of the current PR's scope.

However, the current PR (#37052) is the most recent and comprehensive implementation that combines three improvements. The older PR #12721 appears to address only one of the three features being added in the current PR.

Also found:

• fix(tui): expand pasted text placeholders literally (PR fix(tui): expand pasted text placeholders literally #33700) - relates to the paste expansion functionality

The current PR (#37052) appears to be the main implementation PR. You may want to check if PR #12721 was superseded or merged, and whether #33700 should be referenced alongside this new PR.

[github-actions]

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