feat(banner): hide zsh install tip when plugin is already loaded

[EmojiPati]

Summary

When `forge` is launched from a shell that has already sourced the zsh plugin, the `TIP: For the best experience, use our zsh plugin!` banner is noise — the user already has it configured. Detect that at runtime and skip the encouragement block.

Context

The generated zsh plugin and theme scripts already write `_FORGE_PLUGIN_LOADED` and `_FORGE_THEME_LOADED` timestamps so `forge zsh doctor` can introspect them (`crates/forge_main/src/zsh/plugin.rs:47`, `crates/forge_main/src/zsh/plugin.rs:58`). However the assignments were not `export`ed, so the markers stayed shell-local and were invisible to any child `forge` process. That made runtime detection from inside forge itself impossible.

Changes

• `crates/forge_main/src/zsh/plugin.rs`: prefix both `_FORGE_PLUGIN_LOADED` and `_FORGE_THEME_LOADED` assignments with `export` so they propagate into the environment inherited by invoked forge processes.
• `crates/forge_main/src/banner.rs`: suppress the zsh encouragement block in `display()` when `_FORGE_PLUGIN_LOADED` is set. Adds `is_zsh_plugin_loaded()` modeled after `is_vscode_terminal()` in `crates/forge_main/src/vscode.rs:9-16` — a small `std::env`-backed check with an `EnvGuard`-based test covering both set and unset cases.

Behavior

Scenario	Tip shown?
Plugin not installed	Yes (unchanged)
Plugin installed and `.zshrc` sourced	No (new)
CLI / non-interactive mode (`-p`)	No (unchanged)

Testing

```bash
cargo test -p forge_main --lib banner::
cargo test -p forge_main --lib zsh::plugin::
cargo clippy -p forge_main --lib --tests -- -D warnings
```