Add shared AI assistant context files

.cursor/rules/project-context.mdc

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+---
+description: Project layout, build commands and working agreements
+alwaysApply: true
+---
+
+See [AGENTS.md](mdc:AGENTS.md) for project layout, build and test commands, code
+style and working agreements. That file is the canonical context; this rule only
+points at it.

.github/copilot-instructions.md

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+See [AGENTS.md](../AGENTS.md) for project layout, build and test commands, code
+style and working agreements. That file is the canonical context; this one only
+points at it.

AGENTS.md

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+# Agent context
+
+Shared context for every AI assistant used on this repository: Cursor, GitHub
+Copilot, Claude, Gemini and the Xcode extension itself. The other instruction
+files (`CLAUDE.md`, `GEMINI.md`, `.github/copilot-instructions.md`,
+`.cursor/rules/`) point here, so this file is the single place to edit.
+
+## Project
+
+GitHub Copilot for Xcode: a macOS app plus Xcode source editor extension.
+Requires macOS 13+, Xcode 14+ and a GitHub account.
+
+Targets, and where the code lives:
+
+- `Copilot for Xcode` — host app and settings UI. Implemented in `Core/HostApp`.
+- `EditorExtension` — sandboxed Xcode source editor extension. Forwards editor
+  content to the XPC service and writes the result back.
+- `ExtensionService` — background process where the features live. Implemented
+  in `Core/Service`.
+- `CommunicationBridge` — keeps the host app and extension talking to
+  `ExtensionService` across the sandbox boundary.
+- `Core` and `Tool` — Swift packages holding most of the logic.
+
+`DEVELOPMENT.md` has the full description; read it before restructuring targets.
+
+## Build and test
+
+Node and `npm` must be on the system path, because an Xcode run script resolves
+them with a very limited `PATH`:
+
+```sh
+sudo ln -s `which npm` /usr/local/bin
+sudo ln -s `which node` /usr/local/bin
+```
+
+Local build:
+
+```sh
+cd ./Script
+sh ./uninstall-app.sh    # remove any previous installation
+rm -rf ../build          # clean the build directory
+sh ./localbuild-app.sh   # build a fresh copy
+```
+
+Unit tests run from the `Copilot for Xcode` target. New tests belong in
+`TestPlan.xctestplan`. To exercise the editor extension, run `ExtensionService`,
+`CommunicationBridge` and `EditorExtension` together.
+
+Versions for the app and all targets come from `Version.xcconfig`.
+
+## Code style
+
+SwiftFormat, following the Ray Wenderlich Swift style guide, with 4-space
+indentation (the Xcode default) rather than 2.
+
+## Working agreements
+
+- Keep answers short. One task finished at a time, then say what is still
+  waiting.
+- Split the work by tool: VS Code for running, debugging and small fixes;
+  Claude for long explanations when the error message is already in hand;
+  Perplexity for looking things up on the web; Cursor when many files have to be
+  read and changed at once.
+- Do not start heavy subagents or long research rounds unless asked for.
+- Never update, upgrade or self-update software unless the request says so in
+  that same message. This covers `brew update`/`brew upgrade`,
+  `pip install --upgrade`, npm and yarn upgrades, `mas`, OpenFOAM and Homebrew
+  Python, and app self-updates. Installing a missing package that was asked for
+  is fine; upgrading existing ones is not.
+- Never change git config.

CLAUDE.md

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+See [AGENTS.md](./AGENTS.md) for project layout, build and test commands, code
+style and working agreements. That file is the canonical context; this one only
+points at it.

GEMINI.md

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+See [AGENTS.md](./AGENTS.md) for project layout, build and test commands, code
+style and working agreements. That file is the canonical context; this one only
+points at it.