docs: trim impl detail out of the top-level README

Per mara on hyperhive#1898: the top-level README should not go into
details about impl details or specific subsystems. Trims the
Multi-account Matrix support and GitHub account sections down to a
one-line pointer each.

The matrixAccounts option detail that was only ever documented in the
README (checked: docs/matrix.md is entirely about the host hive-matrix
container, not this per-agent option) moves to docs/tools/matrix.md
instead, next to the account parameter every matrix tool already
takes - the natural home, not a link to a page that never covered it.

docs/tools/matrix.md

@@ -67,6 +67,23 @@ room you haven't read yet.
 
 - `mark_read(room, event_id)` — advance the read receipt
 
+## Multiple accounts
+
+`hyperhive.matrixAccounts` (declared in `agent.nix`) gives an agent
+*additional* matrix identities beyond the hive-internal one — e.g. an
+external-facing account alongside the internal one. Each entry is
+keyed by account name and specifies `tokenFile` (bearer token,
+provisioned out-of-band; basename must start with `matrix-token`),
+`sessionDir` (per-account matrix-sdk sqlite state — crypto keys +
+cache), and an optional `homeserver` (defaults to
+`hyperhive.matrix.url`). The hive-internal account is always named
+`main`, synthesized from `hyperhive.matrix.url` + agent state — this
+option only declares extras, and `main` is a reserved key here.
+Requires `hyperhive.matrix.enable = true`.
+
+Every matrix tool above takes an optional `account` parameter (a name
+from this map) to act as that identity instead of the primary one.
+
 ## Architecture
 
 `hive-matrix-daemon` is a single long-running process (one per agent