fix(copilot): address 9 structural/quality issues in agent spec

Issue #1 (shell cmd over-reliance):
  Every shell block now has an "Else (no terminal)" fallback that
  instructs the agent to infer from visible context only and to
  explicitly label each inference — preventing fake command output.

Issue #3 (evidence anti-leak):
  Added Evidence Integrity section: do not fabricate file paths,
  line numbers, or snippets. If exact line unknown → cite file only.
  If snippet unavailable → state "snippet not verified". If nothing
  concrete → do not write the rule.

Issue #4 (rigid structure kills reasoning):
  Plan template changed from "Output this exact structure" to
  "Use this structure if applicable. Do NOT fabricate content to
  fill fields." Fields with no evidence write INSUFFICIENT EVIDENCE.
  Added TERMINAL ACCESS field so confidence level is explicit.

Issue #5 (no cost model):
  Added Cost Model section: each file adds maintenance cost,
  cognitive load, and contradiction risk. Rule: only add when
  value > long-term cost. This is now also a required field in
  the plan (Cost justification per proposed change).

Issue #6 (staleness not a decision condition):
  Staleness Filter is now a pre-write gate, not just post-run output.
  If a rule is stale-prone within 3 months → move to skill or omit.
  Stale risk is also a required field in the plan per proposed change.

Issue #7 (conflict arbitration incomplete):
  Added Conflict Resolution Priority table (1=copilot-instructions.md
  highest, 5=agents lowest). Each proposed change now includes
  "Authority level: [1-5]" in the plan. Never duplicate across levels.

Issue #8 ("under 100 lines" is arbitrary):
  Changed to "Prefer under 100 lines. If longer, justify why each
  section earns its place." Applied same principle to path
  instructions (<60), skills (<150).

Issue #9 (commit fallback already OK, confirmed):
  No change needed — fallback was already added in prior round.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: YunchuWang