Map "cmux NIGHTLY" to :cmux:

[ShaunMWallace]

What does this PR add or change?

Adds the cmux nightly channel to the existing :cmux: mapping. No new SVG or glyph — this is a mapping-only alias.

cmux ships a separate nightly bundle alongside the stable one:

Channel	Bundle ID	App name reported to sketchybar
stable	com.cmuxterm.app	cmux
nightly	com.cmuxterm.app.nightly	cmux NIGHTLY

Verified on macOS with yabai -m query --windows → "app":"cmux NIGHTLY", and both CFBundleDisplayName and CFBundleName in the nightly bundle's Info.plist are cmux NIGHTLY (uppercase).

-"cmux"
+"cmux" | "cmux NIGHTLY"

This follows the established convention for pre-release channels in this repo — aliasing them onto the stable icon rather than adding a separate glyph:

• :discord: → "Discord" | "Discord Canary" | "Discord PTB"
• :code: → "Code" | "Code - Insiders"
• :google_chrome: → includes "Google Chrome Canary"
• :zen_browser: → includes "Twilight"

Checklist

• SVG file(s) placed in svgs/ — N/A, no new SVG (reuses existing :cmux:)
• Each SVG uses a 24x24 viewBox — N/A, no new SVG
• SVG(s) optimised with SVGOMG — N/A, no new SVG
• Mapping file already exists in mappings/:cmux: with matching name
• Mapping file lists all relevant app name variants in "App Name 1" | "App Name 2" format
• Tested locally — pnpm run validate passes (591 mappings / 641 SVGs). Since there is no new glyph to inspect visually, I verified by running pnpm run build and diffing the generated dist/icon_map.sh against the published v2.0.71 release asset: the only difference is the intended one line.
• No related icon request issue

291c291
<    "cmux")
---
>    "cmux" | "cmux NIGHTLY")

Preview

No preview attached — no glyph changes, the nightly channel simply resolves to the existing :cmux: icon.

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