chore: remove cmux upgrade shims

[arzafran]

What this does

Programa stops reading settings, config files and environment variables left over from when it was called cmux. Anyone who upgraded from cmux was migrated long ago, so current users see no change. Someone jumping straight from an old cmux install would start with default settings.

Summary

Removed:

• The one-time cmux* to programa* UserDefaults migration (Sources/ProgramaApp.swift).
• ~/.config/cmux/settings.json, ~/.config/cmux/cmux.json and project cmux.json lookups, including the trust-store migration branch.
• The cmuxOnly socket mode alias (parser, known modes, schema enum).
• CMUXTERM_* and CMUX_* env fallbacks in the CLI and Bonsplit debug log, the com.cmuxterm.app.debug exclusion, cmux- socket name prefixes, the old wrapper header match, the palette history key rewrite.
• The shell integration fallback to a cmux binary (bash, zsh, fish).
• Tests and docs for the above. Restore notes in docs/removed/cmux-upgrade-shims.md.

Kept on purpose:

• cmuxPortBase / cmuxPortRange: they are the live keys current users store port settings under, not shims.
• "protocol": "cmux-socket" in the capabilities reply (wire value).
• The Sparkle keychain account name in scripts/sparkle_generate_keys.sh (the signing key lives under it).
• Attribution text and test fixtures.

Test plan

• check-structural-budgets.py, check-v2-contract.sh, check-settings-docs.py pass locally.
• programa build and programa-unit build-for-testing succeed locally; bash -n, zsh -n, fish -n clean on the integration files.
• CI unit, UI and socket suites pass.