feat: add SPINE_ORG env var to override org file path

Add SPINE_ORG environment variable support to scripts/run. When set,
the value is passed as spine-org-file via --eval before loading spine.el.

Usage:
  SPINE_ORG=~/my-books.org ./scripts/run
  SPINE_ORG=/work/personal-local/spine/sample-books.org ./scripts/run localhost:9090

When unset, defaults to ~/spine/books.org (existing behavior).

docs/specs/2026-06-24-org-file-env-var-design.md

@@ -0,0 +1,49 @@
+# Parameterize spine.org Path via Environment Variable
+
+## Problem
+
+The org data file path (`spine-org-file`) is hardcoded to `~/spine/books.org` in
+`spine.el`. Users who want to point the app at a different org file must manually
+construct the full `emacs --eval` command line instead of using the `scripts/run`
+convenience script.
+
+## Design
+
+Add `SPINE_ORG` environment variable support to `scripts/run`. When set, the
+script passes the path to Emacs via `--eval` before loading `spine.el`, matching
+the existing pattern for `spine-host` and `spine-port`.
+
+### Interface
+
+```bash
+# Default (same as today): ~/spine/books.org
+./scripts/run
+
+# Custom path, default port 8080
+SPINE_ORG=~/my-books.org ./scripts/run
+
+# Custom path with host and port
+SPINE_ORG=$(pwd)/sample-books.org ./scripts/run localhost:9090
+```
+
+### Affected files
+
+- **`scripts/run`** — add SPINE_ORG env var check before launching Emacs
+- **`spine.el`** — no changes needed (already uses `spine-org-file` defvar)
+- **Tests** — no changes needed (already set their own `spine-org-file`)
+
+### Implementation
+
+In `scripts/run`, after the HOST/PORT argument parsing block (line 18), add:
+
+```bash
+if [ -n "${SPINE_ORG:-}" ]; then
+  EMACS_ARGS+=(--eval "(setq spine-org-file \"$SPINE_ORG\")")
+fi
+```
+
+### Edge cases
+
+- `SPINE_ORG` unset or empty → ignored, default in `spine.el` applies
+- Path with spaces → quoted inside the Emacs string, handled correctly
+- Relative vs absolute paths → user's responsibility; Emacs resolves relative to default-directory