Clarify Symphony service specification (#61)

#### Context

SPEC.md had ambiguous conformance language and implementation details
that made the service contract harder to port.

#### TL;DR

*Clarify SPEC.md and normalize RFC-style requirement wording.*

#### Summary

- Add normative language and tighten config, workspace, reload, restart,
and user-input wording.
- Move extension config ownership to extension sections.
- Point Codex protocol details at the targeted app-server spec instead
of duplicating payloads.
- Run a careful RFC 2119 terminology pass after mechanical
normalization.

#### Alternatives

- Left attempt/retry redesign and broader safety issues for follow-up
spec changes.

#### Test Plan

- [x] `make -C elixir all`
- [x] `git diff --check HEAD~1..HEAD`
- [x] RFC keyword scan outside code fences

---------

Co-authored-by: Codex <codex@openai.com>

Author: frantic-openai