feat(cli): --offline / JCODE_OFFLINE unified startup network kill switch (#24)

[quangdang46]

What

Adds the missing 'offline mode' switch requested in issue #24. A single flag (or env var) disables every startup network operation:

• Update check (src/update.rs::check_for_update_blocking)
• Install/update telemetry (src/telemetry.rs::is_enabled)
• Provider model-list refresh
  • OpenAI (should_refresh_openai_model_catalog)
  • Anthropic (should_refresh_anthropic_model_catalog)
  • OpenRouter (begin_background_model_catalog_refresh)

Provider API calls during the session itself are NOT affected — this is strictly about startup-time chatter, mirroring pi-mono's PI_OFFLINE.

Usage

jcode --offline run "hello"     # CLI flag
JCODE_OFFLINE=1 jcode             # env var (works for wrappers)

When active, prints once on startup (suppressed by --quiet):
Offline mode: startup network operations disabled (JCODE_OFFLINE=1).

Changes

• src/cli/args.rs: offline: bool global flag.
• src/cli/startup.rs: parse_and_prepare_args translates args.offline || env::var('JCODE_OFFLINE').is_ok() into a single in-process JCODE_OFFLINE=1 env so deep code paths can read it without threading an extra arg. should_spawn_background_update_check short-circuits.
• src/update.rs: check_for_update_blocking returns Ok(None) immediately, logs "Update check skipped".
• src/telemetry.rs: is_enabled returns false after the existing JCODE_NO_TELEMETRY / DO_NOT_TRACK checks.
• src/provider/models.rs: should_refresh_*_model_catalog short-circuit before throttle / cache-fresh checks.
• src/provider/openrouter.rs: begin_background_model_catalog_refresh short-circuits.

Tests

$ cargo test -p jcode --lib offline
test result: ok. 2 passed; 0 failed

• update::tests::check_for_update_blocking_returns_none_in_offline_mode
• telemetry::tests::telemetry_disabled_by_jcode_offline_env

Notes

Future feature work that adds new startup network calls (skill index refresh #16, package update checks #6, extension registry #3) should follow the same pattern: short-circuit at the entry point with if std::env::var("JCODE_OFFLINE").is_ok() { return ... }.