Migrate Device Agent to FlowFuse branding

[knolleary]

Description

Whilst we've been working through this under the banner of https://github.com/FlowFuse/admin/issues/211, I wanted to dedicated issue for the Device Agent as there are a few pieces to get right in one go and wanted to make sure we tracked them properly.

The plan is to rename the Device Agent's npm module to @flowfuse/device-agent. However we will cross-publish to both the old and new module name for a period of time to ease the migration.

### Tasks
- [x] Update package.json to `@flowfuse/device-agent`
- [x] Update repo secrets - see below
- [x] Update publish workflow to both flowforge and flowfuse modules
- [x] Update workflow to cross publish to both flowforge and flowfuse containers
- [x] Update README to reflect renaming
- [x] Update to use `/opt/flowfuse-device` as the default config dir - with fallback support for old name
- [x] Add startup log message if found to be installed as `@flowforge..` pointing to the new module name
- [x] Update docs on migration

We need to update some repo-level secrets:

• NPM_PUBLISH_TOKEN_FLOWFORGE - the token for publishing to old npm org
• NPM_PUBLISH_TOKEN - the token for publishing to the new org
• DOCKER_HUB_PASSWORD_FLOWFORGE - the password for the old dockerhub account
• DOCKER_HUB_PASSWORD - the password for the new dockerhub account

Epic/Story

No response

[knolleary]

All tasks complete pending a review and merge of the main doc updates