(MOT-4011) feat(harness): report context compactions

[ytallo]

Refs MOT-4011

What changed

Adds durable context-compaction counts to harness metrics for each active session path and for the complete root/descendant tree. The metrics reader now includes custom transcript entries and counts only records whose custom type is compaction.

Why

Compaction frequency is useful diagnostic evidence for long-session behavior while the latest context snapshot remains a separate per-generation signal.

Impact

This is an additive V1 contract field. It does not change generation behavior, scoring, or promotion policy.

Validation

• cargo test --manifest-path harness/Cargo.toml --locked --all-targets
• cargo clippy --manifest-path harness/Cargo.toml --locked --all-targets -- -D warnings
• cargo fmt --manifest-path harness/Cargo.toml --all -- --check
• schema golden tests

[github-actions]

skill-check — worker

0 verified, 60 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.

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