Keep authoring guidance out of published SDK docs

[1fanwang]

The docs sync currently includes the contributor style guide, so it would publish authoring instructions as an SDK documentation page. Keep that guidance outside the synced tree while retaining its docs/** scope and the root guide's pointer.

Fixes #2758

Testing

Reproduced the reporter's directory-copy boundary locally. The authoring guide was copied before this change and is excluded afterward; every remaining source file is unchanged. The hosted docs sync was not run.

Local source-copy check

$ mkdir -p .scratch/docs-sync
$ rsync -a --delete --exclude=.validation/ --exclude=developer-docs/ docs/ .scratch/docs-sync/
$ test ! -f .scratch/docs-sync/AGENTS.md
# Before: exit 1
# After: exit 0

The existing documentation extractor produces the same examples, and the TypeScript snippet validator passes.

[Copilot]

Copilot review overview

🟢 Approval recommended

The relocation prevents publication while retaining discoverability and scope.

Review effort: Balanced
Findings: None

What changed in this PR

Relocates contributor-only documentation guidance outside the published docs/ tree while preserving its scope.

Changes:

• Moves the style guide under .github/instructions with docs/** targeting.
• Updates the root guidance link.

File	Description
AGENTS.md	Points to the relocated style guide.
.github/​instructions/​docs-style.instructions.md	Adds front matter preserving docs/** scope.

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