docs: use Gemini reviewer path in harness

[limaronaldo]

Description

Substitutes the active Grok reviewer path in the harness with Gemini Flash 3.5.

Changes Made

• Updates active harness README guidance from Claude Code + Grok Build to Claude Code + Gemini Flash 3.5.
• Updates review-gate.sh comments and user-facing handoff text to use REVIEWER_CLI=gemini and a Gemini CLI non-interactive example.
• Records the substitution in live harness progress and the active sprint log.
• Adds a Review Canvas and final post-review artifact for the process-critical harness script change.

Testing

• rtk gemini --help — PASS; CLI installed and supports -m/--model.
• rtk gemini -m gemini-3.5-flash "Return exactly OK" — BLOCKED by local Google account licensing (SUBSCRIPTION_REQUIRED), so Gemini could not be used for this post-review run.
• rtk bash -n docs/harness/bin/review-gate.sh — PASS.
• rtk git diff --check — PASS.
• rtk bash docs/harness/bin/doctor.sh — PASS.
• rtk bash docs/harness/bin/sensors.sh — PASS, full gate (make ci + pr-title-policy + harness doctor).
• rtk bash docs/harness/bin/review-gate.sh post reviewer-cli-gemini-substitution --review-file docs/harness/reviews/2026-06-22-reviewer-cli-gemini-substitution-v3-post.md — PASS.

Notes

Gemini is now the documented reviewer path. The exact Gemini Flash 3.5 model ID remains configurable until the local Google account has a valid license and the model alias can be confirmed.

[huly-for-github]

Connected to Huly®: ENGRA-153