AI Feature Orchestrator: Custom Agents (2/6), Fixes AB#3534742

[shahzaibj]

Part 2: Custom Agents

Adds 6 custom agents that power the feature orchestration pipeline.

Agent	Purpose
feature-orchestrator	Conductor coordinates Design Plan Backlog Dispatch Monitor
codebase-researcher	Deep exploration with call chain tracing, invariant discovery
design-writer	Design specs with agent implementation notes, open questions
feature-planner	Quality-driven PBI decomposition
pbi-creator	ADO work item creation with batched askQuestion
agent-dispatcher	Dispatches PBIs to Copilot coding agent

Also updates copilot-instructions.md with pipeline documentation and .gitconfig for design-docs clone.

PR 2 of 6. Merge after Skills (#392).

Fixes AB#3534742

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