refactor: reorganize modules and update prune-lock API

[vic]

• Move allfollow.nix to modules/prune-lock/ directory
• Convert dendritic.nix to dendritic/ directory with default.nix
• Update prune-lock API from command string to program derivation
• Add _nothing.nix as example pruning app that does nothing
• Update allfollow.nix to use writeShellApplication pattern
• Rename check-flake to check-flake-file for clarity
• Add standalone prune-lock package

BREAKING CHANGE: prune-lock.command option replaced with prune-lock.program

In preparation for #19

[Copilot]

Pull Request Overview

This PR refactors the module organization and updates the prune-lock API from accepting command strings to program derivations. The key changes include moving modules to better organized directories, updating the prune-lock interface to be more type-safe, and providing clearer naming for check operations.

• Module reorganization: moved allfollow.nix to prune-lock/ directory and converted dendritic.nix to a directory structure
• API change: replaced prune-lock.command (string) with prune-lock.program (derivation) for better type safety
• Enhanced tooling: added standalone prune-lock package and renamed check-flake to check-flake-file for clarity

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
modules/write-flake.nix	Updates prune-lock API usage and adds standalone prune-lock package
modules/prune-lock/allfollow.nix	Relocated allfollow module with updated API using writeShellApplication
modules/prune-lock/_nothing.nix	New example pruning application that performs no-op copy
modules/options.nix	Updates prune-lock option from command string to program derivation
modules/dendritic/default.nix	New directory structure entry point for dendritic modules
modules/dendritic.nix	Removed old dendritic module file
modules/default.nix	Updates import paths for reorganized modules
modules/allfollow.nix	Removed old allfollow module file

Comments suppressed due to low confidence (1)

modules/prune-lock/_nothing.nix:4

• The name 'cp' is misleading as it suggests this is the system cp command. Consider using 'nothing-pruner' or 'noop-pruner' to better reflect its purpose as an example pruning app.

  name = "cp";