feat: make catalog add idempotent across all catalog families (#4505)

[mnriem]

Summary

Addresses #4505. Today every catalog add command fails when a matching entry already exists, which forces workflow authors to add their own duplicate-detection/remove-then-add logic around a command that may run repeatedly.

This makes catalog add idempotent, implementing the feature assessment's recommended Option A — "idempotent no-op for equivalent entries":

• Identical rerun → successful no-op (exit code 0), so the same catalog add is safe inside a re-runnable workflow.
• Same identity but different flags → conflict (exit code 1). We deliberately do not silently overwrite priority/install permissions — the user is told to remove the entry first. (Overwrite is Option B, explicitly out of scope for the recommended option.)
• New entry → added, exactly as before.
• Existing URL/config validation and safety failures are preserved unchanged.

Scope — all six catalog families

Command	Identity key	Equivalence compares
specify extension catalog add	catalog name	url, priority, install_allowed, description
specify preset catalog add	catalog name	url, priority, install_allowed, description
specify integration catalog add	catalog url	name (or none supplied)
specify workflow catalog add	catalog url	name (or none supplied)
specify workflow step catalog add	catalog url	name (or none supplied)
specify bundle catalog add	source id or url	policy, priority

Implementation notes

• The URL-identity class methods (IntegrationCatalog/WorkflowCatalog/StepCatalog.add_catalog) now return "added" / "unchanged" so the CLI can report the no-op; a same-URL add with a different --name raises a conflict.
• bundler.commands_impl.catalog_config.add_source now returns (CatalogSource, status).
• Extension/preset CLI commands compare the requested flags against the stored entry inline and either no-op or error.

Tests & docs

• Updated the former "duplicate rejected" tests to the new semantics and added no-op + conflict coverage per family (extension, preset, integration, workflow, step, bundle).
• Documented the idempotent behavior in the five reference docs (extensions, presets, workflows, integrations, bundles).
• Full relevant suites pass locally: 5147 passed, 5 skipped.

Family checklist

• extension catalog
• preset catalog
• integration catalog
• workflow catalog
• step catalog
• bundle catalog

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This pull request was authored autonomously by GitHub Copilot (model: Claude Opus 4.8) on behalf of @mnriem.

[mnriem]

Superseded by the upstream PR against github/spec-kit (this fork PR was created automatically against the fork's own main by mistake).