docs(coding-agent): add quickstart guide

packages/coding-agent/docs/quickstart.md

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+# Quickstart
+
+This page gets you from install to a useful first pi session.
+
+## Install
+
+Pi is distributed as an npm package:
+
+```bash
+npm install -g @mariozechner/pi-coding-agent
+```
+
+Then start pi in the project directory you want it to work on:
+
+```bash
+cd /path/to/project
+pi
+```
+
+## Authenticate
+
+Pi can use subscription providers through `/login`, or API-key providers through environment variables or the auth file.
+
+### Option 1: subscription login
+
+Start pi and run:
+
+```text
+/login
+```
+
+Then select a provider. Built-in subscription logins include Claude Pro/Max, ChatGPT Plus/Pro (Codex), GitHub Copilot, Google Gemini CLI, and Google Antigravity.
+
+### Option 2: API key
+
+Set an API key before launching pi:
+
+```bash
+export ANTHROPIC_API_KEY=sk-ant-...
+pi
+```
+
+You can also run `/login` and select an API-key provider to store the key in `~/.pi/agent/auth.json`.
+
+See [Providers](providers.md) for all supported providers, environment variables, and cloud-provider setup.
+
+## First session
+
+Once pi starts, type a request and press Enter:
+
+```text
+Summarize this repository and tell me how to run its checks.
+```
+
+By default, pi gives the model four tools:
+
+- `read` - read files
+- `write` - create or overwrite files
+- `edit` - patch files
+- `bash` - run shell commands
+
+Additional built-in read-only tools (`grep`, `find`, `ls`) are available through tool options. Pi runs in your current working directory and can modify files there. Use git or another checkpointing workflow if you want easy rollback.
+
+## Give pi project instructions
+
+Pi loads context files at startup. Add an `AGENTS.md` file to tell it how to work in a project:
+
+```markdown
+# Project Instructions
+
+- Run `npm run check` after code changes.
+- Do not run production migrations locally.
+- Keep responses concise.
+```
+
+Pi loads:
+
+- `~/.pi/agent/AGENTS.md` for global instructions
+- `AGENTS.md` or `CLAUDE.md` from parent directories and the current directory
+
+Restart pi, or run `/reload`, after changing context files.
+
+## Common things to try
+
+### Reference files
+
+Type `@` in the editor to fuzzy-search files, or pass files on the command line:
+
+```bash
+pi @README.md "Summarize this"
+pi @src/app.ts @src/app.test.ts "Review these together"
+```
+
+Images can be pasted with Ctrl+V (Alt+V on Windows) or dragged into supported terminals.
+
+### Run shell commands
+
+In interactive mode:
+
+```text
+!npm run lint
+```
+
+The command output is sent to the model. Use `!!command` to run a command without adding its output to the model context.
+
+### Switch models
+
+Use `/model` or Ctrl+L to choose a model. Use Shift+Tab to cycle thinking level. Use Ctrl+P / Shift+Ctrl+P to cycle through scoped models.
+
+### Continue later
+
+Sessions are saved automatically:
+
+```bash
+pi -c                  # Continue most recent session
+pi -r                  # Browse previous sessions
+pi --session <path|id> # Open a specific session
+```
+
+Inside pi, use `/resume`, `/new`, `/tree`, `/fork`, and `/clone` to manage sessions.
+
+### Non-interactive mode
+
+For one-shot prompts:
+
+```bash
+pi -p "Summarize this codebase"
+cat README.md | pi -p "Summarize this text"
+pi -p @screenshot.png "What's in this image?"
+```
+
+Use `--mode json` for JSON event output or `--mode rpc` for process integration.
+
+## Next steps
+
+- [Using Pi](usage.md) - interactive mode, slash commands, sessions, context files, and CLI reference.
+- [Providers](providers.md) - authentication and model setup.
+- [Settings](settings.md) - global and project configuration.
+- [Keybindings](keybindings.md) - shortcuts and customization.
+- [Pi Packages](packages.md) - install shared extensions, skills, prompts, and themes.
+
+Platform notes: [Windows](windows.md), [Termux](termux.md), [tmux](tmux.md), [Terminal setup](terminal-setup.md), [Shell aliases](shell-aliases.md).