feat: add processing activity timeline

[windzu]

Summary

• replace separate reasoning and tool result cards with one ordered, turn-scoped activity timeline
• keep active work expanded, automatically collapse terminal turns, and show the elapsed processing time in the collapsed summary
• derive readable command, file, search, MCP, planning, and subagent labels locally from normalized event metadata
• persist completed, failed, cancelled, and interrupted turn states with their duration
• split multi-tool Codex exec envelopes into separate live activities so live and restored histories stay consistent

Why

Windy's existing tool events were individually accessible but difficult to follow as one processing flow. The transcript did not provide a prominent running state or a compact completion summary, and newly completed turns did not persist their elapsed time.

The new presentation uses the structured events Windy already receives. It does not make an additional model request and does not consume extra model tokens to summarize activity.

User impact

• running turns show a visible, expanded Working… activity section
• tool activity uses descriptions such as Ran npm test, Edited src/ui/MessageListRenderer.ts, and Searched the web for ...
• completed activity automatically collapses to summaries such as Worked for 1m 42s
• completed activity remains manually expandable, including after switching conversations or restarting Obsidian
• failed, cancelled, and interrupted turns retain an accurate terminal state instead of leaving running tool indicators behind

Validation

• npm run typecheck
• npm test — 94 tests passed
• npm run build

Screenshots

Pending in this draft. The active local Obsidian installation currently includes the unmerged pasted-image work from PR #36, so this branch was not deployed over it with a main-based build.

Closes #37