feat(opencode): V2 commands.pre-execute alongside V1 trigger

[filipeforattini]

Third V1→V2 shim in the series (#70 #72 #75). Adds SessionEvent.Command.PreExecute (live waterfall event, type session.next.command.pre_execute) carrying { command, sessionID, arguments, parts } and wires EventV2Bridge.waterfall into the command dispatch next to the existing V1 plugin.trigger('command.execute.before', ...).

What this PR does

• Adds Command.PreExecute as a sibling namespace to Tool, carrying command/arguments/parts.
• The command dispatch now runs a waterfall with a single listener that delegates to the V1 trigger so existing plugins keep working unchanged.
• Listeners return { parts } to rewrite the prompt payload; throwing short-circuits the command.

Pattern

Same shape as #70 / #72: waterfall returns the (possibly transformed) result, V1 listener falls back to the mutated shared value when no V2 plugin exists, and V2 plugins get right of last say when they do.

Out of scope

• agent.pre-system for experimental.chat.system.transform
• permission.request for permission.ask
• experimental.session.compacting / experimental.compaction.autocontinue / experimental.text.complete (all emit)

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