Record compaction token savings in ledger step events (#143)

[RjyavardhanSingh]

Problem

fixes: #143

When context_compaction rewrites a prompt via _compact_messages, the
compaction happens silently. There is no record of how many tokens it
removed, so you cannot tell whether compaction helped or how much context
was saved.

Solution

Plumb compaction metadata (tokens_before, tokens_after,
tokens_saved) from _compact_messages through to the ledger step
event, following the existing data path:

The compaction field is only present when compaction actually
occurred — no change to the common (non-compacted) path.

Step event with compaction

{
  "kind": "step",
  "node_type": "llm",
  "usage": {"input": 80, "output": 20},
  "compaction": {"tokens_before": 195, "tokens_after": 119, "tokens_saved": 76}
}

COMPACTION RESULTS

Input messages sent: 8
Messages after compact: 5 (3 duplicate assistant msgs dropped)

Tokens BEFORE compaction: 195
Tokens AFTER compaction: 119
Tokens SAVED: 76
Saving percentage: 39.0%

LLM usage reported: input=120, output=15
Cost (micro-USD): 1650