docs(server): design + beads for opt-in Axum server codegen

Adds docs/planning/server-codegen.md capturing the agreed design:
trait-at-the-seam emission for a user-selected subset of operations,
no scaffold file, three CLI commands (list / add / remove), TOML
[server].operations as source of truth.

Beads issues created:
- sot umbrella, depends on phases 1-6
- f8q P1 server list CLI
- lyo P2 config schema + selector resolution
- 56o P3 server add / remove CLI
- jih P4 trait + response-enum emitter
- cdx P5 router factory + extractors + model pruning
- 9ek P6 snapshot tests + smoke test

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lightsofapollo