chore: migrate flake to the purescript-overlay canon, add AGENTS.md

[Unisay]

Moves the dev shell to the shared purescript-overlay canon (purs 0.15.16, spago 0.21.0, Lua 5.1), off the easy-purescript-nix setup the rest of the forks have already left. The CI was brought onto the canon in #3, so this is the flake half plus the AI-agent instructions.

AGENTS.md + CLAUDE.md:

• AGENTS.md is the one instruction file the coding agents read natively (Codex, Cursor, Copilot, Gemini CLI, Aider): the build/test/lint commands, the Lua 5.1 constraints, the FFI parenthesisation rule, and the toolchain pins.
• CLAUDE.md is a one-line @AGENTS.md import so Claude Code reads the same file instead of a second copy that drifts out of sync.

It also drops the dead .eslintrc.json and package.json left over from the JavaScript build.

Tooling and docs only, no src/ change, so there is no tag or package-set bump.

Verification

nix develop -c ./scripts/build compiles the fork on the overlay shell (writes dist/Effect_Console.lua) and luacheck with --std lua51 --no-unused-args src/ is clean locally.

[Copilot]

Pull request overview

Migrates the project’s Nix flake-based developer environment from easy-purescript-nix to the shared purescript-overlay canon, and adds standardized AI-agent instructions for consistent build/lint workflows across coding agents.

Changes:

• Update flake.nix/flake.lock to use purescript-overlay and pin the canonical PureScript + Spago + Lua 5.1 toolchain.
• Add AGENTS.md (and CLAUDE.md import) describing project constraints and common commands for coding agents.
• Remove leftover JavaScript-era tooling files (package.json, .eslintrc.json).

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
package.json	Removes unused legacy JS build tooling metadata.
.eslintrc.json	Removes unused legacy ESLint configuration.
flake.nix	Switches dev shell to purescript-overlay canon and adds nixConfig cache settings.
flake.lock	Updates lockfile to reflect the overlay migration and refreshed inputs.
AGENTS.md	Adds agent-readable build/lint/toolchain + Lua 5.1 constraints documentation.
CLAUDE.md	Imports AGENTS.md so Claude Code reads the same instructions.

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