Rewrite AGENTS.md for integration subpackage architecture

[Copilot]

AGENTS.md documented the old AGENT_CONFIG dict-based process (edit dict, update help text, modify release scripts, add case statements). Since #1924 shipped the integration subpackage architecture, AI agents following this guide produce incorrect changes.

Removed

• Supported Agents table — redundant with integration class definitions; drifts
• 7-step guide — references AGENT_CONFIG, release ZIP scripts, --ai-skills flag, case statements — all gone
• Agent Categories / Directory Conventions — derivable from requires_cli and registrar_config.dir on each class

Added

• Architecture overview — integrations/<key>/ subpackage structure, INTEGRATION_REGISTRY as single source of truth
• Base class decision tree — MarkdownIntegration / TomlIntegration / SkillsIntegration / IntegrationBase
• Step-by-step guide — create subpackage → register → add scripts → test → optional overrides
• Real code examples pulled from existing integrations (Windsurf, Gemini, Codex, Copilot)

Kept unchanged

• About Spec Kit and Specify, Command File Formats, Argument Patterns, Devcontainer section

A minimal new integration now looks like:

class WindsurfIntegration(MarkdownIntegration):
    key = "windsurf"
    config = {"name": "Windsurf", "folder": ".windsurf/", "commands_subdir": "workflows",
              "install_url": None, "requires_cli": False}
    registrar_config = {"dir": ".windsurf/workflows", "format": "markdown",
                        "args": "$ARGUMENTS", "extension": ".md"}
    context_file = ".windsurf/rules/specify-rules.md"

Plus one import + _register() call in _register_builtins() and two thin wrapper scripts.