refactor(ci): read the CI environment through the funnel

AGENTS.md

@@ -122,10 +122,13 @@ allowed — use an exhaustive `enum` match.
 - Commands emit results through `&mut dyn mergify_core::output::Output`, not
   `println!`. Call `emit` / `emit_json_value` once.
 - **Color goes through `mergify_tui::theme`**, never hardcoded SGR escapes. The
-  theme honors `--color <auto|always|never>` (resolved once via
-  `set_color_choice`), then `NO_COLOR`, then `FORCE_COLOR`/`CLICOLOR_FORCE`, then
-  the TTY. Cursor-movement / erase escapes in the progress renderer are not color
-  and are fine.
+  precedence is `--color <auto|always|never>`, then `NO_COLOR`, then
+  `FORCE_COLOR`/`CLICOLOR_FORCE`, then the TTY, and it is resolved once at the
+  CLI entry point: `resolve_color_choice` in `main.rs` folds the three variables
+  into the `ColorChoice` it hands `set_color_choice`. `mergify-tui` itself reads
+  no environment variable, which is why a consumer crate's tests no longer touch
+  the environment just by rendering a themed line. Cursor-movement / erase
+  escapes in the progress renderer are not color and are fine.
 
 ```rust
 // GOOD

README.md

@@ -213,7 +213,8 @@ These are accepted on every command:
 | `GITHUB_REPOSITORY` | Default `owner/repo` when `--repository` is omitted. |
 | `MERGIFY_API_URL` | API base URL (default `https://api.mergify.com`). |
 | `RUST_LOG` | Fine-grained log filtering; overrides `--verbose`. |
-| `NO_COLOR` | Disable colored output. |
+| `NO_COLOR` | Disable colored output. Must be set to a non-empty value; exported-but-empty counts as unset. |
+| `FORCE_COLOR`, `CLICOLOR_FORCE` | Force colored output when it would otherwise be off. Same non-empty rule, and `NO_COLOR` wins over both. All three apply to `--color auto` only. |
 | `MERGIFY_INSTALL_DIR`, `MERGIFY_VERSION` | Install-script target directory / pinned version. |
 
 ## Exit codes

crates/mergify-ci/Cargo.toml

@@ -30,7 +30,6 @@ url = { workspace = true }
 [dev-dependencies]
 mergify-test-support = { path = "../mergify-test-support" }
 tempfile = { workspace = true }
-temp-env = { workspace = true, features = ["async_closure"] }
 tokio = { workspace = true }
 wiremock = { workspace = true }