Let the image build see the dependency Cargo patches

The rename pass vendored a patched sieve-rs and pointed Cargo.toml's
[patch.crates-io] at vendor/sieve-rs. .dockerignore ignores everything and
re-includes a short list that did not have vendor on it, so the image build
had no such directory and stopped at

    failed to load source for dependency `sieve-rs`
    failed to read /build/vendor/sieve-rs/Cargo.toml

CI could not have caught that: it builds from a checkout, where the
directory is simply there, and only the image build has a context to prune.
The first that was known about it was a tag that had already been pushed.

So: vendor is re-included, and tools/fork/context-check.py now asserts the
thing that was quietly assumed -- every path a [patch] section names exists
and survives .dockerignore. It runs beside the other fork checks and takes
no toolchain.

Also, the comments in .dockerignore started with // , which Docker does not
read as a comment: they were patterns that happened to match nothing. They
are # now.

tools/fork/context-check.py

@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2026 Coffey Labs
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Every path Cargo patches has to be in the image's build context.
+
+Cargo.toml's [patch.crates-io] can point at a directory in this repository,
+and the Dockerfile builds from a context that .dockerignore prunes to almost
+nothing. Those two facts met on 2026-09-23: a vendored, patched sieve-rs
+landed, CI stayed green -- it builds from a checkout, where the directory is
+simply there -- and the release build failed on
+
+    failed to load source for dependency `sieve-rs`
+    failed to read /build/vendor/sieve-rs/Cargo.toml
+
+after a tag had already been pushed. This is seconds, and it runs beside the
+other fork checks rather than waiting for a release to find out.
+"""
+
+import re
+import sys
+from pathlib import Path
+
+root = Path(__file__).resolve().parents[2]
+
+
+def patched_paths(manifest: Path) -> list[str]:
+    """Directories named by a [patch...] section's `path = "..."` entries."""
+    out, in_patch = [], False
+    for line in manifest.read_text().splitlines():
+        stripped = line.strip()
+        if stripped.startswith("["):
+            in_patch = stripped.startswith("[patch")
+            continue
+        if not in_patch:
+            continue
+        m = re.search(r'path\s*=\s*"([^"]+)"', stripped)
+        if m:
+            out.append(m.group(1))
+    return out
+
+
+def allowed(dockerignore: Path) -> set[str]:
+    """The first path segment of every re-inclusion rule."""
+    keep = set()
+    for line in dockerignore.read_text().splitlines():
+        stripped = line.strip()
+        if stripped.startswith("!"):
+            keep.add(stripped[1:].strip("/").split("/")[0])
+    return keep
+
+
+def main() -> int:
+    paths = patched_paths(root / "Cargo.toml")
+    if not paths:
+        print("no patched paths to check")
+        return 0
+    keep = allowed(root / ".dockerignore")
+    bad = []
+    for p in paths:
+        top = p.strip("/").split("/")[0]
+        if top not in keep:
+            bad.append((p, top))
+        elif not (root / p).is_dir():
+            bad.append((p, None))
+    for path, top in bad:
+        if top is None:
+            print(f"Cargo.toml patches {path}, which does not exist", file=sys.stderr)
+        else:
+            print(
+                f"Cargo.toml patches {path}, but .dockerignore does not re-include {top!r}:\n"
+                f"  the image build would not see it, and cargo would fail on it.\n"
+                f"  Add `!{top}` to .dockerignore.",
+                file=sys.stderr,
+            )
+    if bad:
+        return 1
+    print(f"build context includes every patched path: {', '.join(paths)}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())