feat(completion): handle symlinked directories in file completion

[sudo-tee]

This should resolve #336

[Copilot]

Pull request overview

Adds support for mentioning/adding files located under symlinked directories within the current working directory (fix for #336), primarily by changing how “inside cwd” is determined so that logical (non-resolved) paths can be accepted.

Changes:

• Update util.is_path_in_cwd to allow relative paths that stay logically within cwd without resolving symlinks.
• Adjust file completion to build an absolute path without explicitly resolving symlinks.
• Add unit tests for util.is_path_in_cwd.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
lua/opencode/util.lua	Adds a “logical path” check path for relative paths to support symlinked directories.
lua/opencode/ui/completion/files.lua	Changes how completion items compute full_path used when adding a file to context.
tests/unit/util_spec.lua	Adds new test cases for util.is_path_in_cwd.

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