feat(codeagent): support custom commands and subagents

[CarlJi]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 26.50528% with 1184 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@353b527). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
internal/context/github_context.go	31.58%	414 Missing and 17 partials ⚠️
internal/modes/custom_command_handler.go	0.00%	406 Missing ⚠️
internal/command/definition.go	27.41%	130 Missing and 5 partials ⚠️
internal/command/merger.go	40.84%	69 Missing and 15 partials ⚠️
internal/command/context_processor.go	64.93%	35 Missing and 19 partials ⚠️
internal/code/claude_docker.go	0.00%	21 Missing ⚠️
internal/config/config.go	20.83%	15 Missing and 4 partials ⚠️
internal/workspace/repo_manager.go	0.00%	13 Missing ⚠️
internal/code/gemini_docker.go	0.00%	10 Missing ⚠️
internal/agent/agent.go	0.00%	7 Missing ⚠️
... and 2 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #308   +/-   ##
=======================================
  Coverage        ?   13.19%           
=======================================
  Files           ?       47           
  Lines           ?    10500           
  Branches        ?        0           
=======================================
  Hits            ?     1385           
  Misses          ?     8981           
  Partials        ?      134           

Flag	Coverage Δ
unittests	13.19% <26.50%> (?)

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[CarlJi]

考虑到当前PR越来越大，且是开关试设计，不会响应现有逻辑。计划先合并，再小步迭代。