🤖 feat: enable mux testing mux via isolated roots

[ibetitsmike]

This enables running multiple mux desktop instances on the same machine without collisions by scoping both mux state and Electron userData per root.

What changed

• Added a supported CLI flag --mux-root <path> (alias --root) that sets MUX_ROOT early and strips itself from argv for subcommand routing.
• Desktop now derives Electron userData from the chosen mux root (<muxHome>/user-data) so the single-instance lock + localStorage are effectively per-root.
• Legacy migration (~/.cmux → ~/.mux) is skipped when using a custom root to avoid side effects.
• Added a built-in skill: mux-testing-mux.
• Added debug command palette actions:
  • Debug: Launch Test Instance (creates <muxHome>/instances/<id> and spawns a new mux process with --mux-root)
  • Debug: Delete Test Instances… (cleanup)

How to use

• Launch a second instance manually:
  • mux --mux-root /tmp/mux-test-1
• Or from the desktop app command palette:
  • Debug: Launch Test Instance

Validation

• make static-check
• Added unit tests for argv parsing, userData resolution, and config sanitization.

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Generated with mux • Model: openai:gpt-5.2 • Thinking: xhigh • Cost: $14.25

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[ibetitsmike]

@codex review

Addressed the port-collision concern by clearing apiServerPort in the isolated test instance config so spawned instances bind to a random available port.

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[ibetitsmike]

@codex review

Also updated consumeMuxRootFromArgv to treat --mux-root/--root --help (and other flag-like values) as a missing-value error, matching typical CLI parser behavior.

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[ibetitsmike]

@codex review

Also updated test-instance spawning to override MUX_USER_DATA_DIR to <newRoot>/user-data, so isolation still works even if the parent mux process was launched with a custom userData dir.

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