Revert NixOS-focused Linux release packaging changes

[Copilot]

This change backs out the most recent release/install updates and restores the prior release artifact shape. The revert removes the Linux-specific launcher/payload packaging path that had been introduced for NixOS compatibility.

• What changed

  • Restored release packaging to publish a single ai binary in each release archive.
  • Removed Linux-only bundling of .ai-real and .ai-lib runtime payload files from the release workflow.
  • Reverted installer handling that staged hidden runtime payload files during upgrade/install.
  • Removed the added installation notes describing the bundled Linux payload layout and NixOS behavior.

• Release workflow

  • Switched archive creation back to packaging dist/sea/ai directly as ai.
  • Dropped the extra asset-level smoke test that exercised the packaged launcher layout introduced by the reverted commit.

• Installer/runtime layout

  • Restored the previous extraction path:

    tar -xzf "$download_path" -C "$TMP_DIR"
    tmp_bin="$TMP_DIR/ai"

  • Removed support for installing adjacent hidden runtime files (.ai-real, .ai-lib).

• Docs

  • Deleted the NixOS/Linux payload guidance added to docs/install.md so the install docs match the restored release artifact format.