feat(cli): add cwd option for parent directory context

[piersolenski]

Summary

Adds a cwd option to sidekick.cli.send(), sidekick.cli.prompt(), and sidekick.cli.render() that allows overriding the working directory used when resolving context variables like {file} and {position}.

Use Case

When working in nested project directories (e.g., monorepos or related projects in a parent folder), it's useful to send file context relative to the parent directory rather than the current project root. This allows AI tools to understand the broader project structure.

Changes

• Added optional cwd field to sidekick.cli.Send and sidekick.cli.PromptOpts
• Updated Context.get() to accept optional cwd parameter
• Enhanced path normalization in location.lua to handle custom cwd correctly
• Backward compatible: defaults to current working directory when not specified

Example Usage

-- Send file with parent directory as context root
require("sidekick.cli").send({
  msg = "{file}",
  cwd = vim.fn.fnamemodify(vim.fn.getcwd(0), ":h")
})