docs(sandboxes): add policy advisor guide

[johntmyers]

Summary

Add a Manage OpenShell docs page for policy advisor, covering how to enable the agent_policy_proposals_enabled setting and how sandboxed agents use policy.local to submit draft policy changes for developer approval.

Related Issue

None.

Changes

• Added docs/sandboxes/policy-advisor.mdx with the policy advisor workflow, proposal mechanics, review commands, and expected audit logs.
• Linked the new guide from the denied-request debugging section in the policies page.
• Moved inference routing after the new policy advisor page in the sandboxes sidebar ordering.

Testing

• mise run pre-commit passes (skipped per docs-only request)
• Unit tests added/updated (not applicable, docs only)
• E2E tests added/updated (not applicable, docs only)
• mise run docs passes
• fern check --warnings reports only existing environment/theme warnings
• git diff --cached --check passes

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable, published docs only)

[github-actions]

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