Allow disabling prestartup auto pip install/upgrade

[caniko]

Summary

Adds a dependency_management config key (with a
COMFYUI_MANAGER_DEPENDENCY_MANAGEMENT env override) to opt out of every
prestartup path that mutates the Python interpreter, so Manager can be
used on top of an externally-managed env without screaming subprocess
errors.

Why

When ComfyUI-Manager runs on an interpreter the user doesn't own — Nix,
Guix, distro Python, locked-down corporate images — every startup ends
the same way:

error: The interpreter at /nix/store/.../python3-3.13-env is externally
managed, [...]
[ComfyUI-Manager] Failed to restore comfyui-frontend-package
Command '[..., '-m', 'uv', 'pip', 'install',
'comfyui-frontend-package==1.42.15']' returned non-zero exit status 2.

The env already has the right packages pinned by whoever produced it;
Manager doesn't need to reinstall them, and on a read-only store it
can't. Today there's no clean way to tell it to stand down — it tries
pip → python -m uv → standalone uv and only gives up after all
three fail, which produces a noisy startup and a confused user.

What changes

When dependency_management = off (or
COMFYUI_MANAGER_DEPENDENCY_MANAGEMENT=off):

• PIPFixer.fix_broken() returns immediately. No torch rollback, no
  opencv alignment, no comfy uninstall, no comfyui-frontend-package
  reinstall.
• The unified dependency resolver block in prestartup_script.py is
  skipped before it reaches uv pip install.
• Per-node pip install -r requirements.txt in
  execute_lazy_install_script is skipped.

Read-only operations (pip list, pip freeze, pip show) still run,
so the UI inventory, cm-cli listings, and security_check keep
working. User-initiated installs via cm-cli install and the UI install
buttons are deliberately not gated — those are explicit actions and
shouldn't be silently swallowed.

The default is on, which preserves today's behavior bit-for-bit. No
existing user is affected unless they flip the switch.

Configuration

config.ini:

[default]
dependency_management = off

Or, without touching the config:

COMFYUI_MANAGER_DEPENDENCY_MANAGEMENT=off comfyui --enable-manager

The env var wins over the config key. Accepted "off" values are off,
false, 0, no, disabled (case-insensitive); anything else is
treated as on.

Test plan

• Default (key/env unset) — startup behavior matches main
• dependency_management = off — ComfyUI starts with
  --enable-manager on a Nix-managed env without subprocess errors
• COMFYUI_MANAGER_DEPENDENCY_MANAGEMENT=off overrides
  dependency_management = on
• pip list/freeze/show paths still work (UI inventory,
  security check)
• cm-cli install <node> still installs (CLI not gated)

Notes

Branched off manager-v4 (4.2.1). +46/-1 across two files:
comfyui_manager/common/manager_util.py and
comfyui_manager/prestartup_script.py.