Track upstream integration foundation -- Stage 1 (804cd10)

[brianluby]

Summary

Track upstream commit 804cd10 (PR github#1925) -- Stage 1 of the "Agents to Integrations" architectural migration.

This is a deferred item. No action needed now, but worth tracking for future upstream convergence.

What it introduces

A new src/specify_cli/integrations/ package with:

• IntegrationBase ABC and MarkdownIntegration base class -- each AI agent becomes a class instead of a config dict entry
• IntegrationOption dataclass for per-integration CLI options
• IntegrationManifest with SHA-256 hash-tracked install/uninstall -- enables safe removal that only deletes files the integration created (skips user modifications)
• INTEGRATION_REGISTRY -- empty global dict, populated by later stages as integrations are migrated
• 34 tests at 98% coverage across 9 test classes (~460 lines)

Current status

• Zero callers -- nothing in upstream imports or uses this yet
• Stage 2 (Copilot migration onto these primitives, PR Stage 2: Copilot integration — proof of concept with shared template primitives github/spec-kit#2035) is still open
• Purely additive: 4 new files, 0 existing files modified

Why defer

1. No feature or correctness value today -- the registry is empty and nothing calls into it
2. The conceptual model references CommandRegistrar.AGENT_CONFIGS from agents.py, which our fork doesn't have (our CommandRegistrar is inline in extensions.py)
3. Stage 2 is not yet merged upstream, so the full picture isn't clear

Why track

1. Cherry-pick is mechanically clean (all new files, zero conflicts) -- can be brought in at any time
2. If upstream merges Stage 2+, this becomes a prerequisite for staying in sync with the agent architecture
3. Understanding the direction helps inform whether we should align our AGENT_CONFIG dict / CommandRegistrar structure

Files

File	Lines	Purpose
src/specify_cli/integrations/__init__.py	+34	Registry + public API exports
src/specify_cli/integrations/base.py	+215	IntegrationBase ABC, MarkdownIntegration, IntegrationOption
src/specify_cli/integrations/manifest.py	+265	SHA-256 file tracking for safe install/uninstall
tests/test_integrations.py	+460	34 tests, 98% coverage

Upstream reference

• Commit: github/spec-kit@804cd10
• PR: Stage 1: Integration foundation — base classes, manifest system, and registry github/spec-kit#1925
• Part of tracking issue: From Agents to Integrations github/spec-kit#1924
• Author: Manfred Riem (@mnriem)