Allow excluding specific plugins from updateAll

## Summary
Add a per-plugin way to exclude selected plugins from `/pp updateAll`, while keeping them tracked by Plugin Portal for listing, recognition, and manual updates.

Credit to [Zoriot](https://github.com/Zoriot) for the report.

## Problem
Some server owners need to hold back a specific plugin version even when newer marketplace versions exist. Today `/pp updateAll` considers every tracked local plugin and has no per-plugin skip/pin mechanism. The only workaround is to stop tracking the plugin in Plugin Portal metadata, which is not ideal because it also loses normal Plugin Portal management context.

A concrete example is a plugin releasing an update that breaks worlds or server behavior, where the owner still wants to update other plugins safely.

## Proposed behavior
- Add a per-plugin update exclusion or pin flag for locally tracked plugins.
- `/pp updateAll` should skip excluded plugins by default.
- Single-plugin `/pp update ` should still allow a deliberate manual update, or should require an explicit force flag if that is safer.
- The skipped plugins should be shown in updateAll output so operators understand why they were not updated.

## Possible command surface
One of these would work:

- `/pp update ignore ` and `/pp update unignore `
- `/pp pin ` and `/pp unpin `
- `/pp updateAll --include-ignored` for deliberate override

## Implementation notes
- Store the setting with the local plugin tracking data, for example an `updateIgnored` or `pinned` boolean on `LocalPlugin`.
- Preserve backwards compatibility with existing `plugins.json` entries that do not include the new field.
- Filter ignored plugins before building the `/pp updateAll` update plan.
- Keep the plugin visible in `/pp list`, recognition, import/export, and manual update flows.

Start by tracing the LocalPlugin tracking data, existing plugins.json entries, and the /pp updateAll flow. Define how the new per-plugin flag remains backward-compatible, is exposed through the chosen command surface, and is preserved in listing, recognition, import/export, and manual updates. Done means excluded plugins are skipped and explained by updateAll while still remaining tracked and manually updateable.

Written by the indexing model from the issue text.