chore: add README.md

Author: AshotN

README.md

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+# OpenCode Companion for JetBrains IDEs
+
+Integrates the [OpenCode](https://opencode.ai) AI coding assistant into IntelliJ-based IDEs.
+
+## Features
+
+### Server lifecycle management
+
+The plugin can launch, monitor, and stop the OpenCode server for you.
+
+- **Start / Stop** — Start the server from the tool window with a single click. The plugin spawns
+  `opencode serve --port <port>` in the project directory.
+- **Auto-detect a running server** — If OpenCode is already running on the configured port (e.g. started from the
+  terminal), the plugin connects to it automatically without taking ownership of the process.
+
+Once the server is running the plugin connects over SSE (Server-Sent Events) to receive real-time events.
+
+### Session list
+
+The **Session changes** panel lists all root sessions. Each row shows:
+
+- Session title and a short description
+- A spinner while the session is actively running
+- The number of files being tracked in that session
+
+Clicking a session row makes it the active session; the file list updates to show only that session's changes.
+
+### Modified-files list
+
+The **Files** pane shows every file touched by the selected session:
+
+| State    | Visual                     |
+|----------|----------------------------|
+| Modified | Bold filename              |
+| Added    | Green filename             |
+| Deleted  | Red strikethrough filename |
+
+Interact with the list:
+
+- **Double-click** a file — opens a diff view (see below)
+- **Right-click** — context menu with _Jump to Source_ and _Open Diff_
+- **F4** (or IDE's _Jump to Source_ shortcut) — navigate directly to the file in the editor
+
+### Inline diff in the editor
+
+While a session is active, changed lines are highlighted inlined in the editor:
+
+- **Added lines** — green background highlight
+- **Removed lines** — rendered as red inlay blocks above the insertion point, showing the original content that was
+  deleted
+
+_Due to OpenCode limitations, only the last write will be shown._
+
+This feature can be toggled in settings.
+
+### Diff view of modified files
+
+Double-clicking a file in the modified-files list opens a standard JetBrains **diff editor tab** comparing:
+
+- **Left side** — the session baseline (the content before the agent started making changes)
+- **Middle** - The current file on disk
+- **Right side** — The changes the AI had made to the file
+
+### Send selection to TUI
+
+Share code from the editor directly with OpenCode's prompt input:
+
+- Select any code in the editor
+- Use **Send Selection to OpenCode** from the right-click context menu
+- The selection is sent as a fenced code block (with filename and line range) to the active TUI session's prompt input
+  buffer
+
+The text is appended — not submitted — so you retain full control before pressing Enter in the TUI.
+
+You can also send file and folder paths to the TUI by right-clicking them in the project view.
+
+### Open in browser
+
+The **Open in Browser** toolbar button opens the OpenCode web UI in your default browser.
+
+### New session in TUI
+
+The **New Session** toolbar button creates a new OpenCode session and immediately switches the active TUI instance to
+it.
+
+### Permission prompts
+
+When any agent requests permission, a prompt appears at the bottom of
+the panel with three choices:
+
+- **Allow** — permit this one request
+- **Allow Always** — permit this type of request for the rest of the session
+- **Reject** — deny the request
+
+Responses are sent back to the server immediately.
+
+## Requirements
+
+- A JetBrains IDE based on IntelliJ Platform 2023.3 or later
+- [OpenCode CLI](https://opencode.ai/docs) installed and on `PATH` (or the path configured in settings)