Add no-invalid-agent-fields and enum-return-needs-as-const lint rules

[Copilot]

Two friction points identified in the daily rig evaluation: misspelled agent() spec keys are silently dropped at runtime, and bare string literal returns in defineTool handlers widen to string, causing type errors at the schema comparison site rather than the return site.

New rules

rig/no-invalid-agent-fields

Flags unknown keys on the agent({...}) spec object. TypeScript's excess property checking doesn't reliably catch these.

// Flagged — silently ignored at runtime
agent({ instructions2: p`Review the diff.` })

// Valid fields: name, instructions, input, output, model,
//               maxTurns, addons, agents, systemMessage, tools
agent({ instructions: p`Review the diff.` })

Available in both the tokenizer-based standalone linter (lint.js) and the ESLint plugin. No autofix.

rig/enum-return-needs-as-const

Flags bare string literal returns inside handler: bodies that lack as const.

// Flagged — TypeScript widens "stable" to string
handler: ({ risk }) => { return "stable"; }

// Fixed (autofix available)
handler: ({ risk }) => { return "stable" as const; }

ESLint-plugin only — the tokenizer cannot track scope to limit checks to handler: bodies.

Documentation

• references/linting.md: added rule docs with examples for both rules
• SKILL.md: addons: repair() → addons: [repair()] for consistency with the array form; added composition invariants for valid agent fields and the as const requirement

• Fixes [rig-tasks] Daily rig evaluation — 2026-07-28 — 10/10 passed #252