perf(ai): O(1) exchange_with_id via exchange_id_index

[acarl005]

Description

Part of a stack of performance fixes for #9799: opening a large Oz agent conversation (3000+ exchanges) caused a beachball / multi-second load due to several O(n²) hotspots in the restore path.

This PR: AIConversation::exchange_with_id previously performed a linear scan through every exchange on every call. During conversation restoration this function is called once per exchange to look up command-block metadata, making the restore path O(n²) overall.

Adds exchange_id_index: HashMap<AIAgentExchangeId, usize> to TaskStore. The index is rebuilt on all structural mutations (append, insert, remove, exchange removal). exchange_with_id and the new exchange_by_id now return in O(1).

Also fixes remove(): previously it retained entries in linearized_refs but left stale entries in exchange_id_index for the removed task; now it does a full index rebuild.

Linked Issue

Fixes #9799

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

Manually loaded a conversation with 3014 AI exchanges and confirmed the restore time was significantly reduced.

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

[acarl005]

• perf(ai): O(1) exchange_with_id via exchange_id_index #13111 : 2 dependent PRs (#13112 , #13113 ) 👈 (View in Graphite)
• master

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[acarl005]

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[acarl005]

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Overview

This PR adds an exchange_id_index to TaskStore so AIConversation::exchange_with_id can resolve exchanges by ID through a hash map instead of scanning every task. The index is initialized from restored tasks and rebuilt on the structural mutation paths shown in the diff, including inserts, removals, exchange appends/removals, truncation via modify_task, and explicit exchange ID reassignment after fork/hydration flows.

Concerns

• No blocking correctness, security, or spec-drift concerns found in the changed lines. spec_context.md contains no approved or repository spec context for this PR.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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