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>

CHANGELOG.md

@@ -11,6 +11,7 @@
 - Added an initial `flake.nix` for a ready made development environment; see #3578 (@vorburger)
 - Add `--quiet-empty` (`-E`) flag to suppress output when input is empty. Closes #1936, see #3563 (@NORMAL-EX)
 - Improve native man pages and command help syntax highlighting by stripping overstriking, see #3517 (@akirk)
+- Add `--fallback-syntax`/`--fallback-language` to apply syntax highlighting only when auto-detection fails, see #1341 (@Xavrir)
 
 ## Bugfixes
 - Report error when pager is missing instead of silently falling back, see #3588 (@IMaloney)