github: PATs live in swarm bao; the agent fetches them itself

An operator links an agent's GitHub personal access token in the swarm UI
(LinkGithubAccountForm, "link github account" on /agents). swarm-controller's
PUT /api/hives/{hive}/agents/{agent}/github-account stores it at
swarm/agents/<agent>/github-token (swarm_secret_client::github), a flat leaf
under the agent's prefix that the agent's existing read grant already covers:
no policy change, and no list grant, since there is one token per agent.

In the agent, hive-agent-github-token (oneshot + 2-minute timer, as the agent
user, under its own store certificate, ordered before hive-github-notify)
reads that path and writes <state>/github-token, 0600 and agent-owned, the
file the gh wrapper, git credential helper and hive-github-notify already
read. It replaces the file by rename only when the bytes changed and never
deletes it: a hive-written github-token stays until a token is linked in the
swarm UI. It is installed only with a store address and
services.hyperhive.agent.github.enable.

Removed: the dashboard's CR3D3NTIALS page (credentials.html/js/css, its
build entries and H0M3 tile; GITHUB was its only tab), hive-c0re's
dashboard/matrix_accounts.rs with GET/POST /api/github-account,
priv_client::write_agent_github_token, the host socket's
SetAgentGithubToken and `hivectl github set-token`, and hive-priv's
WriteAgentGithubToken with write_agent_state_file, its only caller gone.

Docs: integrations/github.md and swarm/ui.md describe the swarm path,
swarm/credentials.md gains the store-path row, and the hive UI docs,
hivectl docs and security.md's hive-priv table drop the removed pieces.

Closes #4347

hivectl/README.md

@@ -3,7 +3,7 @@
 The operator-facing host CLI. A thin client for the `hive-c0re`
 daemon — speaks the host admin socket protocol (`hive-host-sock`) and
 does not link the daemon crate. Container lifecycle, the approval
-queue, and the matrix/github/gateway verbs all forward to the daemon
+queue, and the matrix/gateway verbs all forward to the daemon
 and need it running; a few (`wg`/`peer-config`, `choom`) work off local
 host state instead.
 
@@ -30,7 +30,6 @@ dispatch:
 - **`forge.rs`** — reconciles an agent's config against its forge
   repo.
 - **`matrix.rs`** — invite a matrix user to the hive Space or a room.
-- **`github.rs`** — per-agent GitHub PAT writes (`set-token`).
 - **`gateway.rs`** — gateway Basic-auth user management (htpasswd).
 - **`wg.rs`** — WireGuard mesh helpers.
 - **`subvol.rs`** — btrfs state-subvolume ops.