ai: batch agent validation

[ovitrif]

This PR updates the shared agent instructions to batch Gradle validation during multi-step or stacked PR work, while still requiring compile, unit tests, and detekt before the final PR update or push for code changes.

Rationale is that with the current setup, work is slowed down too much due to having to compile after every small change and when an AI agent is requested to do many changes, this accumulates.

Risk is minimized by still requiring validation to pass before PR update/push, and by rerunning the narrowest useful check after fixing validation failures.

Description

• Replaces per-edit Gradle checks with a batched validation cadence for multi-step changes, stacked PR surgery, and review follow-up.
• Keeps final compile, unit test, and detekt checks required before pushing code changes.
• Narrows reruns after validation fixes, especially when only test files changed.
• Allows targeted checks earlier when they answer an immediate implementation risk.

Preview

N/A

QA Notes

Manual Tests

N/A

Automated Checks

• git diff --check
• Not run: Gradle checks skipped because this is a documentation-only change to agent instructions.

[jvsena42]

ACK