# 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