`src/contributor.rs` — per-turn message injection

The simplest extension point: one trait, called at the top of every turn, whose answer rides that turn’s request and then vanishes.

Key items

• ContextContributor::contribute(&self, ctx: &ContributorContext) -> Option<Message> — the whole trait.
• ContributorContext { turn, conversation } — the turn about to run and a read-only view of what the model will see; deliberately not Clone.

Behavior notes

• Registered with agent.add_contributor(...); consulted in registration order; messages prepended to the turn’s request.
• Never persisted — not into history, not into pending, not into the audit trail. Stop returning a message and it’s gone next turn.
• A turn deferred to compaction re-consults contributors against the compacted history, with a reserved budget so the fresh messages fit.
• Extras count toward the context estimate (the compaction trigger sees them).

Deep dive: Contributors. Shipped implementation: presets::GoalReminder.