chore: add agent guidelines, editorconfig, and symlink CLAUDE.md

Add AGENTS.md with guidelines for AI coding agents covering lint
requirements, performance expectations, commit conventions, and
changelog practices. Symlink CLAUDE.md to AGENTS.md so Claude Code
picks up the same rules.

Add .editorconfig enforcing LF line endings, final newlines, 4-space
indentation for Rust, and 2-space for config/doc files.

Author: calebdw

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+indent_style = space
+indent_size = 4
+
+[*.{toml,yml,yaml,json,md}]
+indent_size = 2

AGENTS.md

@@ -0,0 +1,53 @@
+# AI Agent Guidelines for PHPantom
+
+This is a Rust-based PHP language server. Performance and memory
+efficiency are critical -- PHPantom is one of the fastest language
+servers available and it must stay that way.
+
+## Before Committing
+
+Always run these checks before considering any change complete:
+
+```bash
+cargo clippy -- -D warnings
+cargo clippy --tests -- -D warnings
+cargo fmt
+```
+
+Clippy runs twice: once for library code, once including tests. Run
+`cargo fmt` after clippy, not before -- clippy fixes can affect
+formatting.
+
+## Contributing Guidelines
+
+Read and follow [docs/CONTRIBUTING.md](docs/CONTRIBUTING.md) for the
+full set of CI checks, testing conventions, and code style rules.
+
+## Key Rules
+
+- **Performance is critical.** Every allocation, clone, and lock
+  matters. Avoid unnecessary heap allocations, prefer `&str` over
+  `String` where possible, and be mindful of hot paths. Do not
+  introduce regressions in startup time or memory usage.
+- **Run the full lint pipeline.** `cargo clippy` and `cargo fmt` must
+  pass with zero warnings before every commit. Do not skip this.
+- **Update the changelog.** When a change affects user-visible
+  behaviour, add an entry under `## [Unreleased]` in
+  `docs/CHANGELOG.md`. Write for end users, not developers. Include
+  `Contributed by @username` with the GitHub username of the author.
+- **Reference issues in commits.** When fixing a GitHub issue, include
+  `Closes #123` in the commit message body.
+- **Prefer single tests.** Run individual tests (`cargo test test_name`)
+  rather than the full suite during development for faster feedback.
+- **Debug root causes.** When investigating a bug, determine the root
+  cause rather than patching symptoms.
+- **Clean commit history.** Use atomic commits, each representing one
+  logical change. No fixup or WIP commits. Use
+  [conventional commits](https://www.conventionalcommits.org/) for the
+  subject line (`feat:`, `fix:`, `docs:`, `refactor:`, `chore:`, etc.).
+  The commit body should explain *why* the change was made, not just
+  what changed. Wrap the body at 80 characters.
+- **Comments only where they add value.** Don't add obvious or
+  boilerplate comments. Do comment tricky logic, non-obvious design
+  decisions, and workarounds. Follow existing conventions.
+  All files must end with a newline.

CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md

docs/index.md

@@ -1,4 +1,4 @@
-# PHPantom -- a PHP language server
+# PHPantom: a PHP language server
 
 <p align="center">
   <img src="assets/spookaphant.svg" alt="PHPantom" width="200" />