Add GCS_DEFAULT_ORG env var to avoid repeating --org on every invocation

[shouze]

Context

--org <org> is a required CLI flag on every invocation of the default command and the query subcommand. Most users only ever query a single organization, so passing --org on every single call is repetitive friction.

Solution

• Read a new GCS_DEFAULT_ORG environment variable as a fallback default for --org.
• --org (the CLI flag) still takes precedence whenever it's explicitly passed.
• When neither --org nor GCS_DEFAULT_ORG is set, exit with a clear, actionable error message (no raw Commander "required option" stack trace).
• Implementation: switch --org from a Commander requiredOption to a regular Option using Commander's native .env("GCS_DEFAULT_ORG") support (precedence CLI flag > env var is handled by Commander itself), then validate presence explicitly in the action handler.
• Update documentation end to end: docs/reference/environment.md, docs/reference/cli-options.md, README.md, docs/getting-started/index.md / first-search.md.

Acceptance Criteria

• --org <org> continues to work exactly as before and always takes precedence over GCS_DEFAULT_ORG when both are set.
• When --org is omitted and GCS_DEFAULT_ORG is set, its value is used transparently (both the default command and the query subcommand, since they share addSearchOptions).
• When neither --org nor GCS_DEFAULT_ORG is set, the CLI prints a clear error to stderr and exits with a non-zero code.
• --help output documents the GCS_DEFAULT_ORG fallback for --org.
• docs/reference/environment.md, docs/reference/cli-options.md, README.md and the getting-started docs are updated to document GCS_DEFAULT_ORG.

Definition of Done

• bun test, bun run lint, bun run format:check, bun run knip, bun run build.ts all green.
• Docs and the VitePress site reflect the new behavior end to end.