mother: preflight python3, document prereqs, route from build handoff

Three reinforcing changes so the next agent's mother setup lands instead
of failing late:

- setup-mother.sh: fail-fast preflight for python3 (geodesic-dome-mcp is a
  python3 script that otherwise installs fine and fails only when invoked).
- MOTHER-SETUP.md: new Prerequisites section — python3 on PATH, and the
  COLIBRI_AUTOSPAWN_RPC_PROMPT boot decision (set = auto-spawn agent on
  boot; unset = quiet token-free boot).
- FREEBSD-BUILD-LANE-HANDOFF.md: pointer to MOTHER-SETUP.md/setup-mother.sh
  so the mother docs are discoverable from the build-lane entry point.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/FREEBSD-BUILD-LANE-HANDOFF.md

@@ -13,6 +13,11 @@ build, boot it, and run the acceptance checks.
 The `x86_64-unknown-freebsd` binaries must be compiled on the FreeBSD host;
 this lane belongs to the FreeBSD agent end to end.
 
+> **Mother-node setup (osa):** standing up the mother MCP host that USB nodes
+> register into is a separate step from this build lane — follow
+> `packaging/mother/MOTHER-SETUP.md` and run `packaging/mother/setup-mother.sh`
+> (check its Prerequisites first).
+
 ## 0. Host evidence to report
 
 Run once and paste into the validation report:

packaging/mother/MOTHER-SETUP.md

@@ -18,6 +18,16 @@ infrastructure.
 | `build-colibri.sh`  | MCP tool: build a colibri crate (branch-allowlisted)              |
 | `colibri-mcp-ssh`   | SSH forced-command wrapper (allowlisted: `""`, `"tools"` only)    |
 
+## Prerequisites
+
+- **`python3` on PATH** — `geodesic-dome-mcp` is a python3 script (standard
+  library only, no `pip install`). `setup-mother.sh` preflights this and aborts
+  if it is missing. Install with `pkg install python3`.
+- **`COLIBRI_AUTOSPAWN_RPC_PROMPT` (a decision, not a dependency)** — the daemon
+  reads this on boot (`crates/colibri-daemon/src/socket.rs`). Set it to have the
+  daemon auto-spawn an agent with that opening prompt on boot; leave it unset for
+  a quiet, token-free boot. Choose deliberately when configuring the daemon env.
+
 ## Architecture
 
 ```

packaging/mother/setup-mother.sh

@@ -62,6 +62,21 @@ ensure_dir() {
     chmod 750 "$1"
 }
 
+# ── 0. preflight ──────────────────────────────────────────────────────
+# Fail fast on hard dependencies before installing anything, so a missing
+# tool surfaces here rather than as a silent runtime failure later.
+
+echo "--- 0. preflight ---"
+if ! command -v python3 >/dev/null 2>&1; then
+    echo "  ERROR: python3 not found on PATH."
+    echo "         geodesic-dome-mcp is a python3 script; without python3 it"
+    echo "         installs fine but fails when the MCP host invokes it."
+    echo "         Install it first:  pkg install python3"
+    exit 1
+fi
+echo "  python3: $(command -v python3) OK"
+echo ""
+
 # ── 1. colibri binaries ───────────────────────────────────────────────
 
 echo "--- 1. colibri binaries ---"