chore(release): pin pluginApi compat baseline in kb.capabilities

[plind-junior]

What you're trying to do

vouch declares openclaw.compat.pluginApi: ">=2026.4.0" in openclaw.plugin.json. The same constraint should surface in kb.capabilities so non-OpenClaw clients can detect compat without parsing the manifest. Drift between the two declarations should fail CI.

Suggested shape

• kb.capabilities gains a host_compat: {openclaw: {pluginApi: ">=2026.4.0"}} block.
• tests/test_capabilities.py asserts the value matches the JSON in openclaw.plugin.json.
• A future host (Cursor, OpenAI Codex) registers its compat range under the same block.

Acceptance

• Bumping the pluginApi range in one file but not the other fails CI with a clear "host compat drift".

Out of scope

• Multi-host compat matrix beyond OpenClaw in v1.