Target - option to append seeds

[TheTechromancer]

Summary

• Add append_seeds option to targets. When enabled, seeds are appended to the default target-based seeds instead of replacing them. This lets users add a few extra seeds without duplicating all targets into the seeds field.
• Add --append-seeds / -as CLI flag to bbctl scan target create
• Add AGENTS.md with testing guidelines and instructions (CLAUDE.md symlinks to it)
• Move test setup instructions from README.md into AGENTS.md

Test plan

• API test (test_append_seeds) — verifies append behavior, no-op when seeds is None, hash differences
• CLI test (test_cli_targetctl_append_seeds) — verifies --append-seeds flag works end-to-end