v1.12 track B.1: codecontext sidecar container + HTTP shim

New /opt/boocode/codecontext/ directory holding the codecontext sidecar
that BooCode's tool wrappers (track B.2) will talk to. No BooCode-side
changes yet — this commit lands the sidecar standalone.

- Dockerfile: multi-stage golang:1.24-alpine → alpine:3.20. Clones
  codecontext at v3.2.1 from github.com/nmakod/codecontext (cgo build for
  tree-sitter bindings), builds the shim alongside (CGO_ENABLED=0).
- shim.go: stdlib-only Go HTTP server wrapping codecontext's stdio MCP
  child. Newline-delimited JSON framing per the MCP transport spec
  (NOT LSP-style Content-Length). 8 POST /v1/* endpoints, one per MCP
  tool, plus GET /health. Child supervised via child.Wait() goroutine
  that os.Exit's on death so the container's restart: unless-stopped
  policy fires (Signal(0) on a zombie returns nil and is not a liveness
  check — discovered during kill-restart testing).
- go.mod: no third-party deps; future Go security advisories don't apply.

docker-compose service: joins boocode_net (no host port), mounts
/opt:/opt:ro (BooCode projects live at /opt/<slug>, not exclusively
under /opt/projects), healthcheck on /health.

Verified: build clean, healthcheck reports healthy ~15s after up,
multi-project queries return valid markdown, target_dir swap works on
subtree paths. Kill-restart cycle completes in ~200ms with one failed
health poll observed (no misleading "ok" during the gap). Memory: 24.6
MiB after 5 search_symbols calls, 5.6 MiB after 30 min idle — codecontext
releases the per-call graph between target_dir swaps, so the shim doesn't
hold the indexed state.

codecontext/Dockerfile

@@ -0,0 +1,40 @@
+# v1.12 Track B — codecontext sidecar container.
+#
+# Multi-stage build: golang:1.24-alpine builder produces two binaries
+# (codecontext from source + our HTTP shim), then a minimal alpine:3.20
+# runtime holds both.
+#
+# No upstream Docker image exists for codecontext. We clone the repo
+# directly because the module path declared in go.mod
+# (github.com/nuthan-ms/codecontext) differs from the GitHub repo URL
+# (github.com/nmakod/codecontext) — `go install` against the GitHub path
+# wouldn't resolve. The tagged v3.2.1 source tree is the same either way.
+
+FROM golang:1.24-alpine AS builder
+WORKDIR /build
+
+RUN apk add --no-cache git ca-certificates build-base
+
+# Build codecontext from the v3.2.1 tag.
+# CGO is required: codecontext binds tree-sitter via cgo.
+RUN git clone --depth=1 --branch v3.2.1 https://github.com/nmakod/codecontext.git /build/codecontext
+WORKDIR /build/codecontext
+RUN CGO_ENABLED=1 GOOS=linux go build -o /build/codecontext-bin ./cmd/codecontext
+
+# Build the shim. Stdlib-only — no go.sum needed.
+WORKDIR /build/shim
+COPY go.mod ./
+COPY shim.go ./
+RUN CGO_ENABLED=0 GOOS=linux go build -o /build/shim-bin ./
+
+# Runtime: alpine matches the build target so codecontext's cgo bindings
+# resolve against the same musl libc.
+FROM alpine:3.20
+RUN apk add --no-cache ca-certificates
+COPY --from=builder /build/codecontext-bin /usr/local/bin/codecontext
+COPY --from=builder /build/shim-bin /usr/local/bin/shim
+
+EXPOSE 8080
+HEALTHCHECK --interval=30s --timeout=5s --start-period=30s \
+  CMD wget -qO- http://localhost:8080/health || exit 1
+ENTRYPOINT ["/usr/local/bin/shim"]