fix: hyperguild brain write requires wing/hall, not legacy type/slug

PR #131 removed WriteNote's legacy brain/knowledge/ fallback branch that
activated when wing/hall were both empty, but never updated the CLI that
was the only caller relying on it. Since `hyperguild brain write` never
sent wing/hall, every invocation of the CLI's most load-bearing brain
command started hard-erroring "wing and hall are required" — a live,
README-documented command broken on every call.

Separately, writeRequest never had a `slug` field (pre-existing, unrelated
to #131): the CLI's slug value was silently dropped server-side even
before this PR, falling back to a timestamp filename.

Fix:
- `hyperguild brain write <wing> <hall> <slug>` replaces the old
  `<type> <slug>` shape. Positional args, mirroring the existing
  `brain query <topic>` pattern and matching writeHallNote's own
  wing/hall/slug parameter order server-side (handler.go's
  brain.NotePath(brainDir, wing, hall, slug)) — no new flags needed since
  wing/hall/slug are all required, not optional filters.
- brainClient.Write now sends {wing, hall, filename, content}, matching
  writeRequest exactly (writeRequest already gained a Filename field in
  #131, so no server-side change was needed — only the CLI was sending
  the wrong shape). This also fixes the dead slug bug: slug now arrives
  server-side as filename instead of being silently dropped.
- Added TestWrite_CLIPayloadRoundTrips in ingestion/internal/api, which
  POSTs the CLI's exact JSON envelope to the real Handler.Write (not a
  mock) and asserts a real file with real wing/hall frontmatter lands on
  disk. cmd/hyperguild and ingestion are separate Go modules (no
  go.work), so this can't be a single cross-module test — instead the
  wire contract is pinned from the server side, closing the gap where
  cmd/hyperguild/http_test.go's TestBrainClient_Write_Success used only a
  hand-rolled httptest mock and stayed green through the wing/hall
  regression.
- Updated cmd/hyperguild/README.md and the 2026-05-03 CLI design spec's
  stale `<type> <slug>` examples.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/api/handler_test.go

@@ -251,6 +251,50 @@ func TestWrite_GeneratesFilenameIfAbsent(t *testing.T) {
 	assert.True(t, strings.HasSuffix(entries[0].Name(), ".md"))
 }
 
+// TestWrite_CLIPayloadRoundTrips exercises the real Handler.Write (not a
+// hand-rolled mock) over an actual HTTP round trip, POSTing the exact JSON
+// envelope cmd/hyperguild's brainClient.Write sends: {wing, hall, filename,
+// content}. cmd/hyperguild lives in a separate Go module from ingestion
+// (no shared go.work), so a Go-level test can't import brainClient
+// directly — this test instead pins the wire contract from the server
+// side and proves a real `hyperguild brain write <wing> <hall> <slug>`
+// invocation writes a real file with real frontmatter under wiki/<wing>/<hall>/.
+// A prior version of this contract test on the client side (in
+// cmd/hyperguild/http_test.go) used only an httptest.NewServer with a
+// hand-rolled mock body, so it stayed green through a regression where the
+// server required wing/hall but the client never sent them (hyperguild#79
+// PR review).
+func TestWrite_CLIPayloadRoundTrips(t *testing.T) {
+	dir, h := setup(t)
+	srv := httptest.NewServer(http.HandlerFunc(h.Write))
+	defer srv.Close()
+
+	payload, err := json.Marshal(struct {
+		Wing     string `json:"wing"`
+		Hall     string `json:"hall"`
+		Filename string `json:"filename"`
+		Content  string `json:"content"`
+	}{Wing: "homelab", Hall: "facts", Filename: "cli-roundtrip", Content: "# CLI roundtrip\n\nReal write.\n"})
+	require.NoError(t, err)
+
+	resp, err := http.Post(srv.URL+"/write", "application/json", bytes.NewReader(payload)) //nolint:noctx
+	require.NoError(t, err)
+	defer resp.Body.Close() //nolint:errcheck
+
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+	var out struct {
+		Path string `json:"path"`
+	}
+	require.NoError(t, json.NewDecoder(resp.Body).Decode(&out))
+	assert.Equal(t, "wiki/homelab/facts/cli-roundtrip.md", out.Path)
+
+	written, err := os.ReadFile(filepath.Join(dir, "wiki", "homelab", "facts", "cli-roundtrip.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(written), "wing: homelab")
+	assert.Contains(t, string(written), "hall: facts")
+	assert.Contains(t, string(written), "Real write.")
+}
+
 // ---------------------------------------------------------------------------
 // POST /ingest
 // ---------------------------------------------------------------------------