refactor: slim flake.nix into nix/ entry files

nix/sources.nix

@@ -0,0 +1,49 @@
+# Named, explicitly-filtered source views of this repo. Each real
+# consumer gets its own filtered derivation used as `src`, rather than
+# an inline filter at the use site. Companion to the rust `cleanSrc`
+# (crane's cargo-source filter — see ./rust.nix).
+{ lib }:
+{
+  # Filtered view of this repo handed to hive-c0re as the `hyperhive`
+  # meta-flake input. Drops files no nix or cargo derivation reads —
+  # shell helper scripts and root-level markdown — so editing them
+  # does NOT change the store path and therefore does NOT force a
+  # rebuild of every agent container.
+  #
+  # Dropped: scripts/, docs/, root-level *.md (README/CLAUDE/TODO/…).
+  # Kept (build needs them): .nix, .rs, Cargo.*, branding/,
+  # frontend/, prompts/, flake.lock. docs/ is deliberately dropped:
+  # it is shipped to agent containers as its OWN narrow meta-flake
+  # input (`hyperhiveDocsSource`, below), threaded through hive-c0re
+  # → the meta flake → `hyperhive.docs.source`. Keeping docs/ out of
+  # THIS source means a doc edit only re-hashes the docs input (a
+  # cheap re-link), not the whole flake source (which would rebuild
+  # every agent container). The `reference-docs` build from `../docs`
+  # stays only for standalone `nix build .#reference-docs` from a
+  # full checkout — the meta path never evaluates it.
+  hyperhiveFlakeSource = lib.cleanSourceWith {
+    name = "hyperhive-flake-source";
+    src = ../.;
+    filter =
+      path: type:
+      let
+        # Repo-relative path (strip the absolute source-dir prefix).
+        rel = lib.removePrefix (toString ../. + "/") (toString path);
+      in
+      !(lib.hasPrefix "scripts/" rel || rel == "scripts")
+      && !(lib.hasPrefix "docs/" rel || rel == "docs")
+      && !(type == "regular" && !lib.hasInfix "/" rel && lib.hasSuffix ".md" rel);
+  };
+
+  # The repo `docs/` tree as a standalone narrow source. Its store
+  # path moves ONLY on doc edits, decoupled from `hyperhiveFlakeSource`.
+  # hive-c0re threads this to the meta flake as the `hyperhive-docs`
+  # input (same pattern as the `hyperhive` input); the harness resolves
+  # `$HIVE_DOCS_DIR` from it via `hyperhive.docs.source`. Evaluated
+  # here on the host where docs/ exists — it cannot be derived from
+  # inside the docs-stripped `hyperhiveFlakeSource`.
+  hyperhiveDocsSource = lib.cleanSourceWith {
+    name = "hyperhive-docs-source";
+    src = ../docs;
+  };
+}