Trim .github/copilot-instructions.md to reduce attention dilution #999

Problem

.github/copilot-instructions.md has grown to ~2,250 tokens (~9KB, 155 lines). This file is injected into every Copilot coding agent session. At 1.5–4.5× the comfortable ceiling for an always-injected instruction file, it is likely causing attention dilution and inconsistent agent behavior.

Evidence (from Flight analysis)

Bloat sources

Misbehavior mechanisms

• Attention dilution — Critical workflow instructions (Team Context, Capability Check) are buried in the middle where LLMs attend weakest. Defensive rules dominate the top.
• Contradictory commands — Three different "create a feature branch" commands across copilot-instructions.md + team.md. Agent picks whichever gets more attention weight that session.
• Over-caution — "STOP and Ask" on >20 files or any deletions may stall legitimate refactors.
• Duplication — ~470 tokens duplicated with .squad/copilot-instructions.md, ~90 tokens with team.md.

Scope

In scope:

• Trim .github/copilot-instructions.md from ~2,250 tokens → ~800–1,000 tokens
• Extract Protected Files content to an on-demand skill (.copilot/skills/protected-files/SKILL.md)
• Consolidate Git Safety to remove internal repetition and resolve the contradictory branch-creation commands
• Compact the Changeset Requirement section
• Reorder remaining sections so essential workflow instructions come first (where attention is strongest)
• Reconcile with .squad/copilot-instructions.md to eliminate duplication

Out of scope:

• Changes to squad.agent.md (separate concern)
• Changes to team.md Git Workflow section (can be a follow-up)
• Changes to decisions.md size (separate concern)

Acceptance criteria

• File is ≤1,000 tokens (measure with a token counter)
• Protected Files content lives in a skill file, referenced by a single-line pointer
• No contradictory branch-creation commands remain
• Essential workflow sections (Team Context, Capability Self-Check, Branch Naming) appear near the top
• Existing tests pass (npm run test)
• Template sync tests pass (if copilot-instructions.md is part of the template sync)
• Copilot coding agent can still correctly: create branches, open PRs, respect protected files, include changesets