refactor: Extract the substitution behaviour used by custom slash commands to its own file so that it can be more easily reused by new features.

[ariane-emory]

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR extracts the substitution behaviour used by custom slash commands into a new function in its own file, so that it can later be reused when we add the opencode expand command in the feat/opencode-expand branch in this reposiory.

[github-actions]

Hey! Your PR title Refactor/shared substitute 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]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• No "Type of change" checkbox is checked. Please select at least one.
• No issue referenced. Please add Closes #<number> linking to the relevant issue.
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