feat(prompt): render reasoning effort next to model in CLI prompt

[EmojiPati]

Summary

Surface the currently configured reasoning effort (e.g. `HIGH`, `MEDIUM`) in the `forge` CLI's own right prompt, matching the zsh rprompt behavior introduced in #3087. Users now see the configured effort regardless of whether forge is invoked through the zsh plugin or directly.

Context

#3087 added reasoning effort to `ZshRPrompt` (`crates/forge_main/src/zsh/rprompt.rs:33`), which renders the `RPROMPT` for users who have the zsh plugin installed. The `forge` CLI's interactive reedline prompt — `ForgePrompt` at `crates/forge_main/src/prompt.rs:35` — renders agent, tokens, cost, and model but was never updated with the same effort segment. So users running `forge` without the zsh plugin (or in CI, via `-p`, etc.) couldn't see it.

Changes

• `crates/forge_main/src/prompt.rs`
  • Adds `reasoning_effort: Option` to `ForgePrompt` and imports `forge_api::Effort`.
  • `render_prompt_right` appends the effort after the model segment using the same rules as the zsh rprompt: `Effort::None` suppressed, `Color::Yellow` when active (tokens > 0) and `Color::DarkGray` otherwise, and the label collapses to its first three characters on terminals narrower than `WIDE_TERMINAL_THRESHOLD` (100 columns).
  • Helpers `term_width()` and `effort_label(effort, width)` isolate the formatting logic.
• `crates/forge_main/src/ui.rs`
  • `prompt()` resolves the effort via `ForgeAPI::get_reasoning_effort` and forwards it to `ForgePrompt` before each reedline prompt cycle.
  • `ForgeConfig`'s env-var source (`crates/forge_config/src/reader.rs`) already resolves `FORGE_REASONING__EFFORT`, so per-session overrides set by the zsh plugin (`shell-plugin/forge.theme.zsh`) are picked up automatically.

Rendered Examples

Terminal width	Effort	Tokens	Output
Wide (≥100)	`Effort::High`	>0	` FORGE 1.5k gpt-4 HIGH` (yellow)
Narrow (<100)	`Effort::Medium`	0	` FORGE gpt-4 MED` (dim)
Any	`Effort::None` / unset	—	` FORGE gpt-4` (unchanged)

Testing

```bash
cargo test -p forge_main --lib prompt::
cargo test -p forge_main --lib
cargo clippy -p forge_main --lib --tests -- -D warnings
```

Three new unit tests in `prompt.rs`:

• `test_render_prompt_right_with_reasoning_effort` — effort renders after the model segment
• `test_render_prompt_right_hides_effort_none` — `Effort::None` is suppressed (regression guard)
• `test_effort_label_narrow_vs_wide` — width-aware label (`MED` vs `MEDIUM`)

Links

• Companion to feat(rprompt): display reasoning effort next to model in zsh rprompt #3087 (zsh rprompt variant)
• Shell plugin integration: `shell-plugin/forge.theme.zsh`