chore: remove dead code and document repository management

[thomasgarrison]

Summary

Cleanup-only PR, no resource changes expected (plan should be empty apart from refresh):

• Delete the empty src/moved.tf left over from a long-completed state migration.
• Remove the try(local.webhooks["embedded"], {}) lookups in repositories_embedded.tf and repositories_templates.tf — no embedded webhook is defined, so the expression always silently resolved to {}. Now it says {} outright.
• README: replace the outdated repository type list (lib-*-rust/svc-*-rust no longer exist) with the actual repository groups, document the managed vs seeded file mechanisms introduced in fix: stop provisioning cspell config #162, and fill in the long-standing 'Adding a new repository' placeholder — including the cross-repo ordering constraint that teams must exist in tf-onboarding before a repository here can reference them.

🤖 Generated with Claude Code

[github-actions]

Terraform Format and Style 🖌success

Terraform Initialization ⚙️success

Terraform Validation 🤖success

Validation Output

Success! The configuration is valid.

Terraform Plan 📖success

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terraform

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration
and found no differences, so no changes are needed.

Pusher: @thomasgarrison, Action: pull_request, Working Directory: ``, Workflow: Terraform