docs: replace deprecated --ai flag with --integration in all documentation

[mnriem]

Summary

The --ai flag was deprecated in favor of --integration in #2218 (see CHANGELOG), but all user-facing documentation still shows the deprecated --ai form. This creates a confusing first-run experience — new users copy-paste deprecated commands and immediately see deprecation warnings.

Affected files

File	Approx. --ai occurrences	Notes
README.md	~17	Quick-start examples, usage reference, --ai-skills mentions
docs/installation.md	~8	All uvx one-liner examples
docs/upgrade.md	~11	Every upgrade command example
docs/local-development.md	~5	Dev workflow examples
CONTRIBUTING.md	1	Manual testing example
extensions/EXTENSION-USER-GUIDE.md	1	--ai-skills reference
docs/_site/*.html	many	Generated output — will be rebuilt after source changes

Related deprecated flags in docs

• --ai <agent> → --integration <agent>
• --ai-skills → --integration-options="--skills" (or dropped where the integration defaults to skills)
• --ai-commands-dir <dir> → --integration generic --integration-options="--commands-dir <dir>"

Repro

grep -rn '\-\-ai ' README.md docs/ CONTRIBUTING.md extensions/EXTENSION-USER-GUIDE.md | grep -v '_site/' | wc -l
# Returns ~43 occurrences in source docs

Expected

All user-facing documentation should use the modern --integration flag. A brief note about the deprecated --ai alias being available for backward compatibility is acceptable in one place (e.g., the CLI reference section of README.md), but all examples and instructions should show the canonical form.

Suggested changes

1. Mechanical rename in source Markdown files: --ai <agent> → --integration <agent>
2. Replace --ai-skills references with --integration-options="--skills" (or remove where default)
3. Replace --ai-commands-dir references with --integration generic --integration-options="--commands-dir <dir>"
4. Update the "Specify AI Agent" section heading in README.md to "Specify Integration" or similar
5. Rebuild docs/_site/ after source changes
6. Leave CHANGELOG.md as-is (historical record)

Out of scope

• Removing the deprecated --ai flag from the CLI itself (backward compat)
• Updating test files (tests/) — those intentionally exercise the deprecated flag path
• Updating src/specify_cli/__init__.py help text and examples (separate code change, or can be bundled)