feat: declare vouch-context engine contract

[claytonlin1110]

What changed

Adds a vouch-context OpenClaw context engine: Python implementation under src/vouch/openclaw/, a JS plugin entry at adapters/openclaw/vouch-context-engine.mjs, vouch openclaw-rpc CLI bridge, contracts.contextEngines: ["vouch-context"] in openclaw.plugin.json, and context_engines on kb.capabilities.

Why

OpenClaw deployments need a declared context-engine contract so prompts can route through vouch and get back a cited synthesis with salience and hot memory already woven in — the natural counterpart to gbrain's gbrain-context engine.

Closes #228.

What might break

No breaking changes for existing .vouch/ directories or kb.* callers.

• No on-disk layout changes.
• No existing kb.* method behaviour changes.
• kb.capabilities gains an additive context_engines field; clients that ignore unknown fields are unaffected.
• OpenClaw users who enable the slot must set plugins.slots.contextEngine: "vouch-context" explicitly; default remains the legacy engine.

VEP

Not required — this is an OpenClaw plugin surface and additive capabilities metadata, not a change to the AKBP object model or review-gate protocol.

Tests

• make check passes locally (lint + mypy + pytest)
• New / changed behaviour has a test (tests/test_openclaw_context_engine.py, tests/test_openclaw_plugin_manifest.py)
• CHANGELOG.md updated under ## [Unreleased]

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[plind-junior]

@codex review

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[claytonlin1110]

@plind-junior please review