Add --fallback-syntax for undetected files (#3617)

* feat(cli): add fallback syntax option

Expose a new fallback syntax CLI option so users can opt into syntax highlighting only when auto-detection fails.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* feat(syntax): apply fallback only after detection fails

Use the fallback syntax only when path and first-line detection fail, preserving existing behavior for detected files and explicit language selection.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* test(cli): cover fallback syntax behavior

Add integration coverage for fallback syntax usage, precedence with --language, and no-op behavior when syntax is already detected; update help snapshots for the new option.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* docs(changelog): document fallback syntax option

Record the new fallback syntax feature in the unreleased changelog section.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

---------

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/config.rs

@@ -38,6 +38,9 @@ pub struct Config<'a> {
     /// The explicitly configured language, if any
     pub language: Option<&'a str>,
 
+    /// The fallback syntax used when auto-detection fails
+    pub fallback_syntax: Option<&'a str>,
+
     /// Whether or not to show/replace non-printable characters like space, tab and newline.
     pub show_nonprintable: bool,