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

hive-host-sock/src/lib.rs

@@ -315,12 +315,6 @@ pub enum HostRequest {
     /// Daemon-side equivalent of `hivectl gateway list-users`; the usernames
     /// come back in [`HostResponse::messages`], one per line.
     GatewayListUsers,
-    /// Write (or overwrite) an agent's GitHub PAT under its state dir, via
-    /// the privileged helper. Daemon-side equivalent of `hivectl github
-    /// set-token`. `token` is resolved + non-empty-validated client-side
-    /// (inline flag or stdin); the daemon just persists it. Read live by
-    /// the agent's `gh` wrapper / git credential helper — no rebuild needed.
-    SetAgentGithubToken { agent: Ident, token: String },
     /// Turn on btrfs qgroup accounting on the agent-state filesystem, via
     /// the privileged helper. Daemon-side equivalent of `hivectl
     /// quota-enable`. Returns advisory lines in [`HostResponse::messages`].