Add installable SmarkForm AI skill specs and wire them into agent docs

[Copilot]

This PR introduces installable skill specifications so external AI tools (opencode and similar) can generate SmarkForm forms consistently using canonical guidance. It also formalizes a docs-first knowledge resolution order and maintenance rules so skill behavior stays aligned with the latest published SmarkForm agent docs.

• Core skill artifact (skills/SmarkForm-Form-Builder.skill.md)

  • Adds a production-focused “Form Builder” skill spec for SmarkForm implementations.
  • Enforces source priority:
    1. deployed docs pages, 2) canonical repo docs sources, 3) repo pointer files.
  • Encodes hard implementation rules for:
    • SmarkForm action/event usage (BeforeAction_* / AfterAction_*)
    • required await myForm.rendered before find()
    • list/template/context semantics
    • root-level security option handling
    • example naming convention (myForm)
  • Adds mandatory output contract with explicit pass/fail compliance checklist reporting.

• Optional advanced skill (skills/SmarkForm-Advanced-Internals.skill.md)

  • Adds a separate internals-oriented spec for complex action semantics, context/target edge cases, and security-sensitive patterns.
  • References the core skill as the canonical source-priority definition to avoid drift.

• Agent-facing docs update (docs/_resources/ai-agents.md)

  • Adds install guide for importing skill files into opencode/similar tools.
  • Documents when to use core vs advanced skill.
  • Adds explicit maintenance policy tying AGENTS doc changes to skill file updates.

• Repository agent policy update (AGENTS.md)

  • Documents skills/ as a maintained agent artifact directory.
  • Extends maintenance checklist to include skills/*.
  • Adds synchronization rule between canonical AGENTS docs and both skill files.

Example (from the new core skill contract):

## Source Priority (mandatory)
1. https://smarkform.bitifet.net/resources/AGENTS/SmarkForm-Forms
2. https://smarkform.bitifet.net/resources/AGENTS/SmarkForm-Usage
3. docs/_resources/AGENTS/SmarkForm-Forms.md
4. docs/_resources/AGENTS/SmarkForm-Usage.md