feat(p10-2): dockerfile-file-v1 chunker (whole-file 1 chunk, symbol <dockerfile>)

Reads entire Dockerfile / Dockerfile.* / *.dockerfile content and emits a
single Chunk with symbol "<dockerfile>", code_lang "dockerfile", line range
1..EOF. Oversize >200 lines splits into line-windows sharing the symbol via
tier2_shared::push_chunks_with_oversize.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-chunk/tests/fixtures/sample.dockerfile

@@ -0,0 +1,5 @@
+FROM rust:1.94-slim AS builder
+WORKDIR /app
+COPY . .
+RUN cargo build --release
+CMD ["/app/target/release/kebab"]