feat(desktop): add configurable branch prefix setting

[0xMudit]

Problem

Fixes #76277

The Desktop app hardcodes posthog/ as the branch name prefix. Users who work across multiple projects or follow different conventions need to customize this prefix.

Changes

• Added branchPrefix setting in GeneralSettings with input UI under a new Git section
• Threaded branchPrefix through settingsStore -> sessionServiceHost -> SessionService -> tRPC -> agent pipeline
• Updated buildLocalAttributionPrompt and buildAppendedInstructions to use dynamic prefix
• Added tests for custom prefix in task-system-prompt, instructions, branchName, and deriveBranchName
• Default remains posthog/ for backward compatibility

Test evidence

All tests pass:

• packages/agent/src/pi/task-system-prompt.test.ts - 4 tests (2 new for prefix)
• packages/agent/src/adapters/claude/session/instructions.test.ts - 16 tests (2 new for prefix)
• packages/core/src/git-interaction/branchName.test.ts - 23 tests (2 new for prefix)
• packages/core/src/git-interaction/deriveBranchName.test.ts - 10 tests (2 new for prefix)
• packages/core/src/mcp-servers/toolDerivation.test.ts - 4 tests (regression check)

TypeScript typecheck passes on agent package.

Disclosure

This PR was developed with the assistance of an AI coding tool.

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