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.

README.md

@@ -15,10 +15,6 @@ which is a centralized API we deliberately don't depend on. We want a
 wallet that talks directly to Koios + Ogmios endpoints — public, self-
 hosted, or whatever the operator points it at.
 
-Also: it's the first Sulkta Rust project — useful as a workout for
-ci-runner's Rust toolchain (per `Sulkta-Coop/infra-repo`
-`spec-ci-runner.md`).
-
 ## Architecture
 
 Three crates in a Cargo workspace:
@@ -66,11 +62,8 @@ Phase 4:
 ## Build
 
 ```bash
-# Local (requires rustc 1.75+)
+# Requires rustc 1.75+
 cargo build --release
-
-# Through ci-runner (preferred — validates the toolchain there)
-ci-runner build aldabra
 ```
 
 ## Run