feat: implement fork agent with parent context inheritance

[zhanba]

Summary

• Adds a new built-in fork agent type that inherits the parent conversation's full message history, enabling context-aware parallel sub-tasks
• Introduces buildForkMessages / buildForkDirective utilities that clone the parent messages (with fresh IDs) and append a structured fork directive as a final user turn
• Refactors getBuiltinSubAgentInfo into a shared helper in @getpochi/common, removing duplicated agent-type switch logic from use-live-sub-task and tool-call-approval-button
• Filters <fork-directive> messages out of the UI formatter (same as <system-reminder>) so they are never rendered to the user
• Passes parent messages through FlexibleChatTransport → createNewTaskMiddleware so fork tasks are seeded with full context at creation time

Test plan

• Run unit tests: bun run test in packages/common
• Verify buildForkDirective wraps directive text correctly
• Verify buildForkMessages clones parent history and appends directive as new user message with fresh IDs
• Verify formatters.ui strips fork-directive messages from rendered output
• Spawn a fork agent from the UI and confirm it receives the parent conversation context

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.90476% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.45%. Comparing base (c413217) to head (61898c5).
⚠️ Report is 385 commits behind head on main.

Files with missing lines	Patch %	Lines
.../common/src/vscode-webui-bridge/types/sub-agent.ts	6.66%	14 Missing ⚠️
packages/common/src/base/agents/fork-messages.ts	75.00%	9 Missing ⚠️
...bui/src/features/tools/hooks/use-live-sub-task.tsx	0.00%	5 Missing ⚠️
.../approval/components/tool-call-approval-button.tsx	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1471      +/-   ##
==========================================
+ Coverage   19.41%   19.45%   +0.04%     
==========================================
  Files         703      705       +2     
  Lines       66504    66565      +61     
  Branches     1632     1588      -44     
==========================================
+ Hits        12909    12950      +41     
- Misses      53316    53336      +20     
  Partials      279      279              

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