Update docs for image-syncer/custom-image-builder naming

[SharpRake]

[ ] Check if this is a typo or other quick fix and ignore the rest :)

Type of change

Documentation Update / Bug Fix

• Renames catalog_syncer/apko_builder to image-syncer/custom-image-builder in the Cosign verification and Custom Assembly GitOps guides, noting that older organizations still use the old names
• Fixes the GitOps guide's GitHub Actions example, which failed at several steps as written
• Corrects the Step 1 chainctl iam identities create command, which used flags that don't exist
• Standardizes the GitOps guide's placeholders and replaces the identity screenshot

What should this PR do?

resolves https://linear.app/chainguard/issue/DOCS-96/update-docs-for-apko-builder-custom-image-builder-naming

Updates both guides to the current signing-identity names and fixes the GitOps guide's workflow example so it runs as written.

Why are we making this change?

Chainguard organizations now provision these identities as image-syncer and custom-image-builder, but the docs only referenced the older catalog_syncer and apko_builder names. Testing the GitOps guide end to end against a real GitHub Actions run also surfaced several errors that stopped the example from working: --claim=k=v flags that chainctl does not accept, shell variables assigned in one run: block but referenced in another, and backslash continuations inside single-quoted jq programs.

What are the acceptance criteria?

• Both guides use image-syncer and custom-image-builder, with the older names given as equivalents rather than removed
• The Step 1 identity command creates an identity a GitHub Actions workflow can actually assume, including on manual runs
• The GitHub Actions example completes a Custom Assembly build when copied and filled in
• Placeholders are consistent within the GitOps guide, and distinguish the GitHub repository from the Chainguard image repository

How should this PR be tested?

Any documentation published to Chainguard Academy is reviewed carefully for accuracy. GUI procedures, API commands, and CLI code snippets in a draft are run and tested thoroughly — by both the author and the reviewer — to confirm they work exactly as written. This helps ensure that readers can follow along and get the same results. See the edu repo's README.

1. Check the preview link and ensure the changes look right
2. On /chainguard/chainguard-images/how-to-use/verifying-chainguard-images-and-metadata-signatures-with-cosign/, confirm the new screenshot renders and the chainctl iam account-associations describe snippets return UIDPs for your organization
3. On /chainguard/chainguard-images/features/ca-docs/custom-assembly-gitops/, run the Step 1 identity command, then copy the GitHub Actions example into a test repository and confirm a manually triggered run completes the build

🤖 Generated with Claude Code

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[matthewhelmke]

LGTM, thank you!