docs: move implementation sections out of the handbook pages

Three handbook pages carried sections an operator never reads, named after
source paths and build internals. They move to pages under 4-implementation,
and each handbook page keeps a short pointer.

- forge.md: the notification poller (what it polls, how it formats,
  de-duplicates and marks notifications read) is now
  agent-tools/forge-notify.md. The forge page keeps a short Notifications
  section.
- matrix.md: the retired hive-side matrix code, the evaluation assertions and
  the fluffychat-web build fixes are now networking/matrix-homeserver.md.
- github.md: the Implementation section (how `gh` and `git push`
  authenticate, and the GitHub poller) is now agent-tools/github.md.

docs/README.md and the agent-tools README list the new pages, and AGENTS.md's
note on mixed operator and implementation docs now says to move the detail
and leave a pointer.

docs/4-implementation/agent-tools/README.md

@@ -18,6 +18,11 @@ behavior. The operator's own tools are in
 - **[forge](forge.md)** — the `hive-forge` Forgejo CLI every agent has
   for issues, PRs, and comments. Not an MCP tool — a binary agents
   shell out to instead of ad-hoc curl.
+- **[forge-notify](forge-notify.md)** — the daemon that wakes an agent on
+  forge activity: what it polls, how it formats and de-duplicates a
+  notification, and when it marks one read.
+- **[github](github.md)** — how an agent's `gh` and `git push` authenticate
+  to GitHub, and its GitHub notification poller.
 - **[forge-cli](forge-cli.md)** — the exhaustive, autogenerated
   flag-by-flag reference for `hive-forge`, kept in lockstep with the
   binary by CI the same way `hivectl-cli.md` is.