feat: add Fable, Codex model, and cmux launch targets

[randlee]

Summary

• Add /sc:fable visible-terminal launching and Fable model aliases to sc-launchpad.
• Add cmux autodetection and --terminal cmux workspace-tab launching.
• Add shared /sc:sol, /sc:terra, and /sc:luna Codex launchers for gpt-5.6-sol, gpt-5.6-terra, and gpt-5.6-luna.
• Keep /sc:codex as a deprecated compatibility alias routed to Terra model and identity generation.
• Update package metadata, documentation, registry data, and regression tests.

Validation

• 46 focused launcher tests passed.
• Package manifest, cross-reference, script-reference, compile, and metadata checks passed.
• Broader suite reached 1,024 passing tests before an unrelated hang in tests/test_sc_ai_cli_templates.py; no test failures were reported.