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>

doc/long-help.txt

@@ -37,6 +37,13 @@ Options:
           name (like 'C++' or 'LaTeX') or possible file extension (like 'cpp', 'hpp' or 'md'). Use
           '--list-languages' to show all supported language names and file extensions.
 
+      --fallback-syntax <fallback-syntax>
+          Set a fallback language for syntax highlighting when auto-detection fails. Unlike
+          '--language', this is only used when no syntax could be detected from filename, custom
+          syntax mappings, or first-line detection.
+          
+          [aliases: --fallback-language]
+
   -H, --highlight-line <N:M>
           Highlight the specified line ranges with a different background color For example:
             '--highlight-line 40' highlights line 40

doc/short-help.txt

@@ -17,6 +17,8 @@ Options:
           Show plain style (alias for '--style=plain').
   -l, --language <language>
           Set the language for syntax highlighting.
+      --fallback-syntax <fallback-syntax>
+          Set a fallback language for undetected syntaxes. [aliases: --fallback-language]
   -H, --highlight-line <N:M>
           Highlight lines N through M.
       --file-name <name>