fix: read version from package.json instead of modifying session.ts (#753)

I am working to simplify the build process. As a first step, update
`session.ts` so it reads the `version` from `package.json` at runtime so
we no longer have to modify it during the build process. I want to get
to a place where the build looks like:

```
cd codex-cli
pnpm i
pnpm build
RELEASE_DIR=$(mktemp -d)
cp -r bin "$RELEASE_DIR/bin"
cp -r dist "$RELEASE_DIR/dist"
cp -r src "$RELEASE_DIR/src" # important if we want sourcemaps to continue to work
cp ../README.md "$RELEASE_DIR"
VERSION=$(printf '0.1.%d' $(date +%y%m%d%H%M))
jq --arg version "$VERSION" '.version = $version' package.json > "$RELEASE_DIR/package.json"
```

Then the contents of `$RELEASE_DIR` should be good to `npm publish`, no?

README.md

@@ -640,7 +640,7 @@ To publish a new version of the CLI, run the release scripts defined in `codex-c
 3. Bump the version and `CLI_VERSION` to current datetime: `pnpm release:version`
 4. Commit the version bump (with DCO sign-off):
    ```bash
-   git add codex-cli/src/utils/session.ts codex-cli/package.json
+   git add codex-cli/package.json
    git commit -s -m "chore(release): codex-cli v$(node -p \"require('./codex-cli/package.json').version\")"
    ```
 5. Copy README, build, and publish to npm: `pnpm release`