docs: centralize coding-agent instructions

[anusbutt]

Outcome

Make AGENTS.md the canonical shared instruction source and reduce CLAUDE.md to an import plus genuinely Claude Code-specific guidance.

• Organize durable shared guidance around architecture, scope, directories, invariants, commands, platforms, and contribution rules.
• Point package identity, version, engines, scripts, and publishing settings to package.json as the source of truth.
• Import shared guidance from Claude Code with @AGENTS.md.
• Retain only /graphkeeper, the Claude skill destination, marked-block ownership, and restart guidance in CLAUDE.md.
• Add a documentation contract test to prevent shared-rule duplication and hardcoded package-version drift.

Removed drift and duplication

• Removed stale starts at version 0.1.0 wording.
• Removed duplicated testing, append-only, diagnostic, inert-data, and Spec-Kit rules from CLAUDE.md.
• Removed the instruction to manually keep both files synchronized; Claude now imports the canonical file.
• Replaced repeated prerequisite versions and release details with links to their existing sources of truth.

Evidence

• npm run typecheck: passed.
• npm run build: passed.
• Contributor documentation tests: 10 passed, 0 failed.
• git diff --check: passed.

Checklist

• Shared rules live only in AGENTS.md.
• CLAUDE.md imports @AGENTS.md.
• Claude Code-specific integration guidance remains intact.
• GraphKeeper runtime, schema, CLI, adapter registry, and ownership behavior are unchanged.
• No generated files or unrelated local files are included.

Compatibility and recovery

Documentation and documentation-contract tests only. No public commands, schema fields, diagnostics, runtime dependencies, generated integrations, or exit codes change.

Rollback is a normal revert of this PR.