Add a setting to disable lstk update checks

[anisaoshafi]

Motivation

lstk asks you to update on every start, with no way to turn it off. With weekly releases, "skip this version" just delays the prompt by a few days.

It is also wrong when another tool installed lstk. mise, Nix, asdf, Scoop and Chocolatey own the binary, so lstk should not replace it. On Nix it cannot: the store is read-only.

Solution

A new [cli] update_check setting, also available as LSTK_UPDATE_CHECK:

• prompt: ask before updating (today's behaviour, still the default)
• notify: print one line and carry on, no waiting for input
• off: no check at all, no output, no network request

Installs owned by another package manager get notify automatically, pointing at that tool instead of lstk update:

> Note: Update available: 0.20.1 → 0.21.0 (installed with mise — run mise upgrade lstk)

lstk update now refuses on those installs and says how to update instead. lstk update --check still works, and running lstk update yourself always checks, whatever the setting says.

The prompt also gained a [N] Don't ask again option, which saves notify to your config. No docs needed to find it.

A bad value is reported and ignored, so a typo cannot stop lstk start.

Also fixes a bug: the non-interactive path built its own options and ignored update_skipped_version. Both paths now share one policy.

Docs

User-facing, needs documenting:

• New config key [cli] update_check (prompt / notify / off, default prompt), and new env var LSTK_UPDATE_CHECK which overrides it.
• lstk update now exits non-zero on installs owned by mise, asdf, Nix, Scoop or Chocolatey, with a new JSON error code UPDATE_EXTERNALLY_MANAGED.
• Those installs see a one-line note instead of the prompt.
• The prompt has a new "Don't ask again" option.

Already updated here: default_config.toml (which had no [cli] section before), docs/structured-output.md, README.md, lstk update --help.

Review

Human review advised. Changes default behaviour for some installs, and adds a public config key, env var, and JSON error code.

Closes DEVX-1029