Add deterministic agent context compiler

[salman1993]

Why

Agent adapters need a deterministic, auditable boundary between durable standing context, prior conversation, and the event authorizing the current turn.

What

• Compile dynamic turns into a Buzz-owned semantic envelope with <ambient-context>, a current <event author="…">, and <delivery>, while standing sections use paired domain tags such as <workspace>, <base>, and <system>
• Keep model-visible framing minimal: section position conveys instruction authority, nonessential source metadata remains manifest-only, and raw events stay available through buzz messages raw
• Publish canonical hashed context-layer manifests, report channel metadata and session-start canvas freshness separately, and incorporate accepted native steers into the completed turn record
• Escape arbitrary standing-context bodies so embedded closing tags cannot spoof boundaries; Desktop diagnostics decode them after parsing and retain legacy prompt-history compatibility

Risk Assessment

Medium — this changes ACP prompt serialization and turn telemetry, but preserves context ordering, keeps legacy observer parsing, and requires no relay storage migration.

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