Implement M7.2 Explanations - queryable timelines and causal summaries

[Copilot]

Adds "why did this happen?" tooling per M7.2: event timelines with causal chains, actor reasoning capture, and CLI queries.

New Module: sim/explanations.py

• ExplanationTracker - records events with causal parent links, metrics snapshots, and actor reasoning
• CausalEvent - tick, message, scope, actor, causality chain
• ActorReasoning - decision context with options considered, scores, and chosen action

CLI Commands

(echoes) timeline 10         # Recent events with causal context
(echoes) explain evt-123     # Event details + causal chain trace
(echoes) why aria-volt       # Actor's recent decision reasoning

Integration

• Agent system captures utility scores and option rankings during decisions
• Faction system captures strategic action reasoning (invest/lobby/sabotage)
• SimEngine exposes get_timeline(), explain_event(), explain_actor()
• Backend interface extended for local/service mode parity

Tests

27 new test cases covering tracker, causal chains, reasoning capture, and CLI commands.

Original prompt

"Implement M7.2 using the gamedev-agent"

Created from Copilot CLI via the copilot delegate command.

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[SorraTheOrc]

@copilot there are many merge conflicts here, rework this patch

[Copilot]

@copilot there are many merge conflicts here, rework this patch

I cannot resolve merge conflicts in this environment - I don't have the ability to pull from the remote, perform rebases, or force push. Since PR #4 (which implemented a different version of M7.2) was merged to main after my branch was created, there are now conflicting changes.

Please resolve the conflicts manually, or close this PR in favor of PR #4 which has already been merged with similar M7.2 functionality.