Bootstrap praxis init with messages and responses starters

[franciscojavierarceo]

Summary

Bootstrap praxis init with a small, truthful first slice aimed at issue praxis-proxy/config-generator#1.

This PR adds:

• a new praxis init <family> <recipe> CLI surface
• real starter generation for:
  • messages protocol
  • messages to-openai
  • responses passthrough
  • responses full-flow
• friendly next-step output after writing praxis.yaml
• quickstart docs for the new bootstrap flow
• unit tests for CLI parsing and file generation behavior

Why

Issue praxis-proxy/config-generator#1 proposes a starter-config CLI that makes Praxis easier to try without starting from a blank YAML file.

This PR establishes the command framework and ships the first recipes from maintained in-repo examples for both Anthropic Messages and OpenAI Responses workflows.

Scope notes

This is still the first slice of the broader epic:

• the framework is general, but the initial shipped families are messages and responses
• interactive setup is still out of scope for v1
• the command currently writes praxis.yaml in the working directory and refuses to overwrite existing files

Validation

• cargo test -p praxis
• cargo run -q -p praxis -- init messages --help
• end-to-end smoke run: cargo run -q --manifest-path server/Cargo.toml -- init messages protocol in a temp dir and verified the generated praxis.yaml
• end-to-end smoke run: cargo run -q --manifest-path server/Cargo.toml -- init responses passthrough in a temp dir and verified the generated praxis.yaml

Closes praxis-proxy/config-generator#1

[shaneutt]

I like the idea, but separating it and letting it soak and grow on its own for a bit will be helpful for cognitive load. I've moved the epic over to a new repository: praxis-proxy/config-generator#1 let's move this over there 👍