docs: scrub remaining internal repo + tooling references

- README.md: drop "first Sulkta Rust project — workout for crafting-
  table's Rust toolchain" paragraph + the `ci-runner build aldabra`
  recipe. Both reference non-public internal infrastructure.
- Dockerfile: drop "Built nightly on the build host (see infra-repo/scripts/
  build.sh)" comment + the `registry.example.com:5000/aldabra/mcp`
  internal image-name advertisement.
- Cargo.toml: drop the comment block referencing the deleted
  `docs/internal-build-rewrites.md` + `ci-runner + the build host + dev
  hosts` internal-builds note. The patch block stands on its
  own.

Dockerfile

@@ -5,10 +5,9 @@
 #   1. builder — rust toolchain, cargo build --release
 #   2. runtime — debian:bookworm-slim, just the binary + ca-certs.
 #
-# Built in CI and
-# published as a container image (aldabra/mcp). Pulled
-# anywhere we want the MCP server available — usually as a sidecar
-# spawned by an MCP client (e.g. Claude Code).
+# Produces an `aldabra/mcp` image with the MCP-server binary baked in.
+# Pulled anywhere the MCP server is needed — usually as a sidecar
+# spawned by an MCP client (Claude Code, etc).
 #
 # Required env at runtime:
 #   ALDABRA_DATA           directory containing mnemonic.age (must