Pure rename ahead of the agent+manager server consolidation: the
per-agent socket dispatcher already hosts the shared dispatch and all
lifecycle handlers, and will absorb the manager-only handlers next, so
`agent_server` becomes a misnomer. No logic change — git mv plus a
mechanical `agent_server` -> `socket_server` rename across refs.
Part of the docs-migration chore (issue #708). Remove GitHub issue
numbers from inline comments, option descriptions, and rustdoc —
these are contextless noise for anyone reading the code without
access to the original discussions. Replace with prose that captures
the same rationale directly.
No functional change. Build still clean (cargo check passes).
