[codex] Document action governance preview gaps

[amavashev]

Summary

Adds human-facing documentation for the remaining v0.1.26 action-governance preview gaps and wires it into the docs navigation.

What changed

• Added a protocol page for the v0.1.26 action-kind registry, action quotas, allow/deny action lists, observe mode, deny/event surface, and quota-counter APIs.
• Linked the new page from the protocol index and sidebar.
• Clarified admin overview optional fields, policy preview filters, tenant observe-mode compatibility filters, dashboard monitoring notes, bulk-action filter behavior, and evaluation guidance.

Why

The YAML specs already described the preview semantics, but the docs did not provide a human-readable map for the action-governance routes and compatibility fields. This made the docs look incomplete even though the current v0.1.25 route coverage was otherwise aligned.

Validation

• npm run build
• action-kinds route coverage check: all four preview routes documented
• git diff --check HEAD~1..HEAD