feat: Optimistically render conversation items before subscription confirms

[charlesvien]

Adds a generic optimisticItems mechanism to the session store so any conversation item type can be displayed immediately before the real event arrives from the main process. Currently used for user messages, but the OptimisticItem union is designed to be extended for future use cases.

1. Add OptimisticItem type and optimisticItems array to AgentSession store
2. Append an optimistic user message when a prompt is sent, before the server round-trip
3. Clear optimistic items when the real session/prompt event arrives or on error
4. Merge optimistic items into the conversation view between confirmed items and queued messages
5. Add useOptimisticItemsForTask hook and store setters (appendOptimisticItem, clearOptimisticItems, appendEventAndClearOptimisticItems)

[charlesvien]

• fix: Reduce agent eagerness to re-enter plan mode mid-task #1208
• feat: Scope oauth tokens by org, replace flat project list with org-keyed map #1210
• feat: Add multi-agent command center grid view #1212
• feat: Add effort level control for Claude sessions #1211
• feat: Optimistically render conversation items before subscription confirms #1209 👈 (View in Graphite)
• feat: Add idle timeout to kill inactive sessions after 15 minutes #1207
• fix: Fix corrupted view redirect, workspace cache flash and diff collapse #1206
• main

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[charlesvien]

Merge activity

• Mar 12, 10:49 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 12, 10:51 PM UTC: Graphite rebased this pull request as part of a merge.
• Mar 12, 10:56 PM UTC: @charlesvien merged this pull request with Graphite.