feat: move handler config out of frame meta to a configuration script

[cablehead]

Move handler configuration from register frame metadata to a record returned by the
handler configuration script. This allows using Nushell code for configuration, making
it more powerful and flexible (e.g. resume_from can use Nushell to locate the right
frame ID).

Config record fields:

• process (required): Handler closure to process frames
• resume_from: Control processing start point (tail/head/id)
• modules: Inline module definitions
• pulse: Heartbeat interval
• return_options: Output frame configuration

Other changes:

• Replace state parameters with environment persistence
• Remove with_env in favor of setting vars in configuration script
• Convert to dedicated per-handler threads, removing thread_pool
• Temporarily remove .process command until use-cases clarify
• add a skeleton astro starlight template to /docs