Add agent task conversation foundation

[klopez4212]

Summary

• Add the core continued agent conversation model, marker event parsing/publishing, and recap/title helpers.
• Add the dedicated agent conversation screen and source-thread navigation primitives.
• Add thread activity polish for agent thinking states and flattened one-level thread replies.

Stack

1. This PR
2. Add channel task surface #1324
3. Add task link cards #1325
4. Gate channel tasks experiment #1275

Snapshots

Task marker and recap

Channel Tasks tab

Tests

• git diff --check origin/main..HEAD
• cd desktop && pnpm check:file-sizes && ./node_modules/.bin/tsc --noEmit
• cd desktop && pnpm test
• cargo test --manifest-path desktop/src-tauri/Cargo.toml deep_link --lib

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