feat(opencode): V2 compaction.pre-compact alongside V1 trigger

[filipeforattini]

Sixth V1→V2 shim in the series (#70 #72 #75 #76 #77 #78). Adds SessionEvent.Compaction.PreCompact (live waterfall event, type session.next.compaction.pre_compact) carrying { sessionID, context, prompt } and wires EventV2Bridge.waterfall into the compaction dispatch next to the existing V1 plugin.trigger('experimental.session.compacting', ...).

What this PR does

• Adds Compaction.PreCompact as a sibling to the existing Compaction.Started/Delta/Ended.
• The compaction dispatch now runs a waterfall with a single listener that delegates to the V1 trigger so existing plugins keep working unchanged.
• Listeners return { context, prompt } to inject context or replace the compaction prompt; throwing short-circuits the compaction.
• The V1 trigger result is mutated with the V2-decided values so downstream code keeps working with a single source of truth.

Pattern

Same as the rest: waterfall returns the (possibly transformed) result, V1 listener falls back to the V1-mutated shared value when no V2 plugin exists, and V2 plugins get right of last say when they do.

Out of scope

• experimental.text.complete → Text.Complete (waterfall, similar pattern)
• experimental.compaction.autocontinue (V1 doesn't mutate, just decides continue/stop)
• Agent loop decomposition (turn flow as events)
• Profiles + bundles

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