feat: add work-item pipeline and worktree policy config

[ksamaschke]

Summary

• add work-item pipeline defaults under autonomy config
• add schema/docs support for worktree branch behavior policy used by process skills

Changes

• ica.config.default.json
  • set autonomy.work_item_pipeline_enabled=true
  • set autonomy.work_item_pipeline_mode=batch_auto
• src/schemas/ica.config.schema.json
  • add autonomy.work_item_pipeline_enabled
  • add autonomy.work_item_pipeline_mode enum
  • add git.worktree_branch_behavior enum (always_new, ask, current_branch)
• docs/configuration-guide.md
  • document work-item pipeline behavior
  • document worktree branch behavior policy and missing-setting ask/persist expectation

Test Plan

• JSON parse validation for updated config files
• full repository test suite (npm test) currently has unrelated pre-existing failures on dev

Breaking Changes

• none