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/printer.rs

@@ -268,7 +268,12 @@ impl<'a> InteractivePrinter<'a> {
             const PLAIN_TEXT_SYNTAX: &str = "Plain Text";
             const MANPAGE_SYNTAX: &str = "Manpage";
             const COMMAND_HELP_SYNTAX: &str = "Command Help";
-            match assets.get_syntax(config.language, input, &config.syntax_mapping) {
+            match assets.get_syntax(
+                config.language,
+                config.fallback_syntax,
+                input,
+                &config.syntax_mapping,
+            ) {
                 Ok(syntax_in_set) => (
                     syntax_in_set.syntax.name == PLAIN_TEXT_SYNTAX,
                     syntax_in_set.syntax.name == MANPAGE_SYNTAX