docs(policies): pivot to the rules registry as the primary install flow

[knhn1004]

Summary

• Per user feedback ("this shouldn't be saying gates are fixed but rather refer to the rules things we have and let them know how to set it up"), the policies guide page now leads with the openagentlock/rules install flow instead of the five hard-coded baseline gates.
• The five built-ins are still documented but now framed as "what ships out of the box" — the typical operator's workflow is now `agentlock rules sync && agentlock rules install `.
• Schema example updated to match the registry's `gate.match` / `gate.evaluate` shape and calls out the RE2 lookahead limitation explicitly.
• Page renamed in docs nav: "Policies and the five gates" → "Policies and rules".
• Cross-references the openagentlock/skills toolkit for the agent-authored-rule path.

Test plan

• Markdown only — mkdocs renders fine locally.
• Reviewer: confirm the new ordering matches the operator journey we want to shape.

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