feat: composable FilterPipeline for chained output filters

[bug-ops]

Parent

Epic #426, Plan: .local/plan/m26.1-output-filtering-improvements.md
Priority: P1 (before Phase 2)

Problem

Current "first matching filter wins" logic cannot combine ANSI stripping + test filtering + deduplication. Multi-command pipelines have undefined behavior.

Design

pub struct FilterPipeline {
    stages: Vec<Box<dyn OutputFilter>>,
}

Registry builds pipelines instead of applying single filter. Hard-coded order: ANSI -> content-specific -> deduplication. Confidence aggregation: Fallback > Partial > Full.

Acceptance Criteria

• FilterPipeline struct with add_stage() and run()
• Registry builds pipelines instead of single filter
• ANSI filter always runs first
• Unit tests: empty, single, multi-stage pipelines
• Integration test: ANSI + test filter composition
• Confidence aggregation logic