feat(tasks): fork tasks with existing context

[richardsolomou]

Problem

Starting a new direction from an existing task should preserve the complete conversation and workspace while allowing both tasks to continue independently. Forked local tasks also need to remain visible while the server list catches up, without adding ambiguous controls to the task header.

Closes #330
Refs #1788

Changes

• Fork local Claude sessions through the SDK transcript-copy API and hydrate the child from flushed logs so completed assistant responses are preserved.
• Keep optimistic fork rows until the server version arrives, with rollback cleanup when provisioning fails.
• Put “Fork task” and “Open source task” in the sidebar right-click menu. Cloud forks still use the persisted run snapshot at a completed turn boundary.

How did you test this?

• Ran the full core suite: 2,467 tests passed.
• Ran the full UI suite: 1,823 tests passed, plus 7 focused Claude adapter tests.
• Ran agent, core, UI, host-router, and desktop typechecks, Biome on changed files, the host-boundary check, and git diff --check.
• Tried live Electron verification on an alternate CDP port, but the dev build did not expose CDP after its dependency rebuild.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 1b3bed5.}