feat(context): add visual selection to context

[sudo-tee]

• Remove selection clear when changing files
• Add add_visual_selection and command
• Add oy mapping to add visual selection to context across buffers

This should fix #248

[Copilot]

Pull request overview

Adds a new workflow for accumulating visual selections into Opencode context across buffers/files (addressing #248), and updates defaults/docs to expose the new command/keymap.

Changes:

• Stop clearing accumulated selections when switching files during context loading.
• Introduce add_visual_selection (context + API command) and bind it to <leader>oy in visual mode.
• Document the new action/keymap in the README and add a shared OpencodeSelectionRange type.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
lua/opencode/types.lua	Adds OpencodeSelectionRange type used for range-based selection APIs.
lua/opencode/context/chat_context.lua	Keeps selections when the current file changes (enables accumulation across files).
lua/opencode/context/base_context.lua	Extends get_current_selection signature to accept an optional range.
lua/opencode/context.lua	Adds context.add_visual_selection() helper to capture and append selections.
lua/opencode/config.lua	Adds default <leader>oy mapping for visual-mode selection accumulation.
lua/opencode/api.lua	Adds :Opencode add_visual_selection command and API entrypoint.
README.md	Documents the new action and keymap.

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