feat(vscode): add factory.cfactoryUrl and make settings.json mutable (#867)

Add the Factory extension setting:
  "factory.cfactoryUrl": "https://cfactory.freundcloud.org.uk"

Previously settings.json was a read-only Stylix-owned store symlink, so the
Factory extension could not persist its runtime keys (e.g. cfactoryToken) and
the user could not hand-edit settings. Now:

- Disable Home Manager's settings.json symlink (home.file ... enable = false).
- Render programs.vscode userSettings (Stylix theme + our keys) to a store file
  and merge it into a MUTABLE settings.json on activation, preserving keys the
  extension/user wrote. Stylix *adds* settings but no longer *owns* the file.

Also fix a latent activation bug in claude-code-lsp.nix: the claudeCodeLspSetup
entry used `exit 0` when claude wasn't on PATH, which aborts the whole activate
script and silently skips every later entry (installPackages, dconfSettings,
vscodeSettings, ...). Replaced with if/else so only that block is skipped.

Verified:
- p620 switched: settings.json is a mutable regular file, colorTheme=Stylix,
  factory.cfactoryUrl clean, cfactoryToken preserved.
- All three hosts build (p620, razer, p510).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: olafkfreund