fix: improve mobile debug and credential handling

[garnetlyx]

Summary

• add an opt-in debug logging setting that writes redacted diagnostics to plugin/log/git-log.md
• mask the stored password/token field in settings and fix migration of the legacy username setting
• make the mobile isomorphic-git adapter throw proper ENOENT errors for missing reads/stat calls instead of returning null-like values or bare objects

Duplicate check

• checked open PRs and recent merged PRs; no open PR covers debug logging, settings password/token masking, or mobile adapter missing-read ENOENT handling
• related merged PR fix: obscure password prompt in isomorphic-git auth #1007 only obscures the isomorphic-git auth prompt, not the stored settings password/token field

Test plan

• pnpm run tsc
• pnpm run svelte
• pnpm run format
• pnpm run lint
• pnpm run build

[Vinzent03]

What is the use case for this specific logging? I would not work on the github token setting at the moment since I would migrate to the new Obsidian secret api to store the token app wide and more securely.