feat(narrator): scene_render tool for visual scene extraction via tool call

[slabgorb]

Summary

Implement scene_render tool to move visual scene description extraction from JSON schema documentation in the narrator prompt to a validated tool call. The narrator creates a scene description (creative judgment), the tool validates and structures it (tier, mood, tags enums), and assemble_turn merges it into the ActionResult.

Acceptance Criteria

1. ✅ scene_render tool accepts subject, tier, mood, tags and returns VisualScene JSON
2. ✅ Tier, mood, and tags validated against their enums
3. ✅ Subject text passed through as-is (narrator's creative judgment)
4. ✅ Narrator prompt documents the tool instead of the JSON field schema
5. ✅ assemble_turn merges into ActionResult
6. ✅ OTEL span with subject text and tier for GM panel visibility

Implementation

• New src/tools/scene_render.rs module with SceneTier/VisualMood/VisualTag enums and validate_scene_render() function
• Tool call parser integration — scene_render match arm calls validate_scene_render()
• ToolCallResults extended with visual_scene field, assemble_turn wires override logic
• Narrator prompt updated to remove visual_scene JSON schema (~100 tokens)
• All error paths emit tracing::warn! events for OTEL observability
• 30 tests covering validation, enum coverage, parser integration, assembler wiring

Design Notes

• Visual scene extraction follows the same tool-based pattern as set_mood/set_intent (story 20-2)
• Subject description is free text from the narrator with 1-100 char constraint (creative judgment unconstrained)
• Enums use #[non_exhaustive] per CLAUDE.md rule chore(sprint): mark stories 1-1 and 1-6 as done #2
• All validators use case-insensitive enum parsing

Test Coverage

• All 30 tests passing, full suite clean (557 tests)
• AC coverage complete: validator, enums, passthrough, wiring, OTEL spans
• Wiring tests verify tool_call_parser integration and assemble_turn output
• Test files in: crates/sidequest-agents/tests/scene_render_story_20_5_tests.rs

Quality Checks

• Build green, no code smells
• All CLAUDE.md rules enforced: #[non_exhaustive], validated constructors, tracing on error paths, doc comments
• Reviewed: all 9 specialists cleared (type design, security, simplicity)

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