feat: add s.path schema helper, improve addon error messages, document tools field

[Copilot]

Three usability gaps surfaced in the daily rig evaluation: missing s.path schema helper, opaque addon validation errors, and undocumented tools spec field.

Changes

s.path schema helper (skills/rig/rig.ts)

New constrained string schema with format: "path" — semantically equivalent to s.url for URIs but for file system paths. Improves readability and hints to the runtime for path-based context resolution.

input: s.object({ filePath: s.path })
// serializes to { type: "string", format: "path" }

Improved addon error messages (skills/rig/rig.ts)

normalizeAddons now reports the failing index and actual type instead of a generic message:

// before
Agent addon entries must be functions.

// after
Agent addon entries must be functions (entry at index 0 is null).

Documentation (skills/rig/SKILL.md, README.md)

• Added tools to the agent(spec) spec fields table — it was entirely absent despite being a first-class spec field
• Added s.path to schema helper reference and examples in both docs

• Fixes [rig-tasks] Daily rig evaluation — 2026-07-24 — 5/5 passed #79