chore: add Copilot support

.github/workflows/copilot-setup-steps.yml

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+name: Copilot Setup Steps
+
+# This workflow defines the setup steps that GitHub Copilot agents will use
+# to prepare the development environment for the cargo-shear project.
+# It preinstalls tools and dependencies needed for Rust development.
+
+on:
+  workflow_dispatch:
+  pull_request:
+    types: [opened, synchronize]
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  push:
+    branches:
+      - main
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+permissions: {}
+
+jobs:
+  copilot-setup-steps:
+    name: Setup Development Environment for Copilot
+    runs-on: ubuntu-latest
+    steps:
+      # Checkout full repo for git history.
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          persist-credentials: false
+
+      - uses: oxc-project/setup-rust@ecabb7322a2ba5aeedb3612d2a40b86a85cee235 # v1.0.11
+        with:
+          cache-key: warm
+          save-cache: false
+          tools: just,cargo-insta,typos-cli,taplo-cli
+          components: clippy rust-docs rustfmt rust-analyzer

AGENTS.md

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+# cargo-shear
+
+Detect and fix unused dependencies, misplaced dependencies, and unlinked source files in Rust projects.
+
+## Development
+
+Run `just ready` after making code changes to ensure code quality and all tests pass.
+
+### Frequent Commands
+
+- `just ready` - Run full quality checks (typos, fmt, check, lint, test)
+- `just fmt` - Format code with rustfmt and taplo
+- `just lint` - Run clippy
+- `just snapshots` - Accept all snapshot test changes
+- `cargo test` - Run all tests
+- `cargo check` - Quick compile check
+
+## Key Modules
+
+- `src/lib.rs` - Core library functionality
+- `src/package_analyzer.rs` - Analyzes package dependencies
+- `src/source_parser.rs` - Parses Rust source files for imports
+- `src/cargo_toml_editor.rs` - Edits Cargo.toml files
+- `src/package_processor.rs` - Processes packages in workspaces
+
+## Testing
+
+- Integration tests: `tests/integration_tests.rs`
+- Test fixtures: `tests/fixtures/`
+- Snapshot tests use `cargo-insta`