docs: pilot split of github.md into operator-facing + collapsed implementation

hyperhive#3902, mara: option (a) - a content pass splitting mixed docs
into operator-facing content plus implementation detail. One file
first, to agree on the split pattern before doing the other ~19.

Went through 3 shapes on review before landing here: a sibling
-internals.md file (mara: clutters the navigation), then two
tree-precedent alternatives damocles raised (subdir+README like
web-ui/; or only split docs with a pre-existing boundary marker,
which would've covered 3-4 of the ~20 flagged docs and left the rest
untouched), then mara's own proposal - a collapsed <details> section
in the same file. Verified empirically (cmark-gfm --unsafe, the
website's own render pipeline) that markdown headings nested inside a
<details> block still parse as real headings with heading-id anchors
once separated from <summary> by a blank line, so anchor links into
the collapsed section keep working.

github.md keeps enabling/provisioning/security up top; its
'Implementation' section is now a <details> block holding what was
briefly a separate github-internals.md (deleted again) - how the
agent's gh/git-push actually authenticate, and the notification
poller's internals. Reverted the two cross-references + the
docs/README.md entry back to pointing at github.md now that the
content lives there again.

Added a short CLAUDE.md note recording the pattern per mara's ask,
including the one real caveat damocles flagged: <details> only
collapses in a rendered browser, a raw-text read (cat, the Read tool)
still sees everything, same as today.

docs/README.md

@@ -55,7 +55,9 @@ declarations.
   [`tools/matrix.md`](tools/matrix.md) for the MCP tool surface and
   `hyperhive.matrixAccounts`.
 - **How do I give an agent a GitHub account (`gh` + `git push`)? How is
-  the PAT injected?** → [`integrations/github.md`](integrations/github.md).
+  the PAT injected?** → [`integrations/github.md`](integrations/github.md)
+  (operator content up top; the `gh`/git-push + notification-poller
+  mechanics are in a collapsed "Implementation" section at the bottom).
 - **What is `/knowledge`? How does the hive-wide knowledge repo sync,
   and how do I contribute a document?** → [`integrations/knowledge.md`](integrations/knowledge.md).
 - **What does `hivectl` do? Provisioning, gateway users, container