docs: actually remove the Agent configuration section, not trim it

Missed mara'\''s reply on this PR before pushing two more commits on top
- she said remove, my first commit trimmed to two bullets, which is
still going into Matrix/GitHub as named subsystems. Deletes the
section outright; the two docs it pointed at (docs/tools/matrix.md,
docs/github.md) are still reachable from CLAUDE.md's reading path and
from docs/tools/README.md, just not named in the top-level README.

README.md

@@ -103,19 +103,6 @@ older/newer channel hits breakage hyperhive's CI doesn't catch.
 For the full list of host and agent NixOS options see the
 **[options reference](https://hyperhive.darkest.space/options/)**.
 
-## Agent configuration
-
-Per-agent config lives in each agent's `agent.nix` (proposed, operator-approved, deployed as git commits). Two accounts an agent can hold, both provisioned mostly outside of nix:
-
-- **A second Matrix identity**, beyond the hive-internal one (e.g. an
-  external-facing account alongside the internal one) — see
-  [`docs/tools/matrix.md`](docs/tools/matrix.md) for the
-  `hyperhive.matrixAccounts` option and setup.
-- **A managed GitHub identity** (`gh` CLI + `git push` over HTTPS) — on
-  by default, provisioned by pasting a PAT into the agent's dashboard
-  credentials tab, nothing to declare in nix. See
-  [`docs/github.md`](docs/github.md) for setup and the disable flag.
-
 ## Operator CLI
 
 `hivectl` is the operator-facing host CLI for ad-hoc administration that