ci: drop build + tests from the pre-push hook

[ShortArrow]

Slims the local git hooks so build + tests are owned by CI, not the pre-push hook.

Why

Pre-push ran dotnet build + dotnet test --filter "Category!=Integration" on every non-docs push. That:

• duplicated the PR CI gate (pr.yml already builds/tests) while diverging from it — the local run has no --locked-mode and a different SDK/OS, so a local pass didn't imply a CI pass and a local failure was sometimes environmental;
• added minutes of latency to every push, and — bypassable with --no-verify — was never a hard gate;
• had grown a ~30-line docs-only skip script that re-implemented CI's changed-files logic locally (a second copy that can drift), just to stay usable;
• was wasteful even on tag pushes (nothing to validate on an already-merged, already-CI'd commit).

What

• Remove .husky/pre-push and its build / test task group from .husky/task-runner.json. The docs-only skip script goes with it.
• Kept: the fast pre-commit CSharpier format check.
• CI (pr.yml) remains the authoritative build/test gate.
• Docs updated: ADR 0025 (§4 supersedes the old pre-push decision, §8, §10), README, and CONTRIBUTING — EN + JP in lockstep.

No product code changes.