feat(updater): delta-only one-liner updater with channel selection

[Chintanpatel24]

Summary of Changes

• Delta-Only In-Place Updater: Reuses persistent cache at ~/.cache/agility-shell/repo using git fetch --prune --tags origin, downloading only new git commits/deltas instead of full re-clones.
• Channel Selection Triage: Prompts the user to choose between:
  1. Latest Release (stable tagged version)
  2. Main branch (bleeding-edge latest commits)
• Non-Interactive Flags: Supports --release / -r and --main / -m for automated updates or direct one-liners.
• Piped Curl Support: Uses /dev/tty for user prompts when executed via curl ... | bash.
• Preserves User Configs: All user settings, custom configs, widgets, themes, and wallpapers are preserved.
• CLI & README Integration:
  • Documented one-liner command in README.md:

    curl -fsSL https://raw.githubusercontent.com/TattvaOrg/agility-shell/main/update.sh | bash

  • Added agl bar and update channels to the documentation.

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