fix(staged): memoize groupByVerb to prevent UI freeze on large sessions

[matt2e]

Summary

• Pre-compute groupByVerb() results for all tool groups in a $derived block, caching both past-tense and present-tense variants
• The template now picks the correct tense via a cheap boolean lookup instead of re-running expensive JSON.parse + string replace operations on every render
• Fixes main thread freezing (heavy String.replace, RegExp.match, string concatenation) when viewing sessions with hundreds of tool calls

Details

groupByVerb() was called directly in a Svelte {#each} template expression, causing it to be re-evaluated on every render cycle. For each tool pair, this triggered:

• parseToolCall() → JSON.parse on every tool call content
• makePathsRelative() → .includes() + .replaceAll() on path strings

For sessions with hundreds of tool calls, this resulted in hundreds of these operations on every single reactivity update.

The fix adds a $derived that pre-computes verb groups for all tool groups when grouped or repoDir change. Since forcePastTense is a boolean, we cache both variants (past and present tense). The template then selects the appropriate cached variant with a simple conditional.

Test plan

• Open a session with many tool calls (50+), verify tool cards display correctly (right verb, right detail text, right tense)
• Verify live sessions show present tense ("Reading", "Running") for pending tool calls
• Verify completed sessions show past tense ("Read", "Ran") for all tool calls
• Verify the modal is noticeably more responsive when scrolling through a long session
• Verify sending a follow-up message correctly switches tense for preceding tool groups

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