fix: deprecation warnings — standardize on gh copilot + Phase 2 plan #925

Follow-up from PR #900 post-merge review

Reviewer: Mal (Lead) via @diberry
Original PR: #900

Findings

1. copilot vs gh copilot inconsistency — Some warnings say both, some say just one. \copilot\ standalone is ambiguous (there's a \squad copilot\ subcommand) and may not exist on PATH. Standardize on \gh copilot\ everywhere.
2. Two unresolved Copilot review threads merged as-is — guidance ambiguity and changeset quote-style nit.
3. No removal timeline — Phase 2 needs explicit version target + migration guide before removal PR.

Action Items

1. Small follow-up PR to standardize all deprecation messages on \gh copilot\
2. Set explicit version target for Phase 2 removal
3. Write migration guide before the removal PR lands