feat: unified collapse bars for thinking, tool calls, and compaction

[mitchellkager]

What

Unifies the collapse/expand UI for thinking blocks, tool calls, and compaction entries into shared top/bottom collapse bars, and adds post-compaction token estimates and elapsed time to the compaction header.

Why

The compaction summary, thinking blocks, and tool call cards each had their own inconsistent expand/collapse affordance (native <details>, separate bars). A single shared pattern is easier to scan and keeps the UI consistent across message types. The compaction header also previously gave no sense of what the compaction did — now it shows the token reduction and how long it took.

Implementation Notes

• lib/message-display.ts: shared collapse bar helpers; compaction gets a single inline header (compaction (size; X → Y tokens) elapsed) expanding via the top bar and collapsing via either bar.
• Thinking blocks converted to the same top/bottom bar pattern with duration in the meta column; tool call cards gain a collapse bottom bar under the existing chevron header.
• lib/session-reader.ts: walks the kept entry chain (firstKeptEntryId → compaction entry's parent) to estimate the post-compaction token count, and derives elapsed time from the entry timestamp gap, so loaded sessions show the estimate inline; live-streamed entries degrade gracefully. systemMessage added to the local CompactionEntry type.
• Refinement pass: top bar chevron moves right (matching tool-call headers), bottom bar chevron points up, compaction header padding aligned with tool-call headers, compaction summary renders as a code-style block.

Testing

• New tests in lib/session-reader.test.mjs for the token-estimate/elapsed-time computation.
• tsc --noEmit, npm run lint, and node --test pass (against the known 7 environment-dependent failures that also fail on main).

[agegr]

Thanks. I'm not going to take this one. It changes behaviour: compaction is now collapsed by default and its summary is shown as raw <pre> text instead of rendered Markdown.

The new "X → Y tokens" and elapsed time in the compaction header are also wrong in cases I tried: a fully emptied kept range shows ~200k instead of ~100 tokens, tool-call arguments are read from the wrong fields (a 40k-character write counts as ~100 tokens), and idle time before a manual compaction is counted as compaction time.