feat: add metadata property bag to context:compaction event

[colombod]

Summary

Adds a metadata property bag to the context:compaction event payload, providing implementation-specific context about why compaction was triggered and the conditions at the time.

Changes

Added metadata dict to the stats payload in _finalize_compaction_with_stats():

"metadata": {
    "reason": "token_threshold_exceeded",
    "threshold": self.compact_threshold,
    "usage_ratio": old_tokens / budget if budget > 0 else 0,
}

• reason — why compaction was triggered (currently always "token_threshold_exceeded" since that's the only trigger path)
• threshold — the configured compaction threshold (default 0.92)
• usage_ratio — the actual token usage ratio that exceeded the threshold

Motivation

The context:compaction event had 14 flat scalar keys with no extensibility mechanism. This follows the metadata property bag pattern documented in CONTEXT_CONTRACT.md (see microsoft/amplifier-core#14), giving consumers structured context about compaction triggers without changing the existing top-level payload shape.

Backward Compatibility

Purely additive — one new key (metadata) added to the existing stats dict. All existing consumers serialize the payload generically (hooks-logging) and will pass through the new field without issue.

Testing

• 21 module-specific tests pass
• 11 errors and 1 failure are pre-existing on main (missing fixture wiring and an unrelated assertion)
• Lint and format checks pass

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