feat(cmd): Allow line ranges in opencode run file attachments

[ewired]

What does this PR do?

Allows specifying line range for the opencode run CLI file attachment option -f. e.g. opencode run "Tell me the exact contents of this line range" -f path/to/file.js:59-66. It will verify whether there is a file with that exact name (for whatever strange reason someone might have that syntax in their filenames) before extracting the line number/range.

How did you verify your code works?

cd packages/opencode && bun dev run "Repeat the exact contents of the attached files" -f README.md:12 -f bunfig.toml -f package.json:11-15

It successfully repeated the given snippets and whole files.

Closes #14193

[github-actions]

Hey! Your PR title Allow line ranges in opencode run file attachments doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.