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-priv-sock/src/lib.rs

@@ -478,22 +478,6 @@ pub enum PrivRequest {
         paused: bool,
     },
 
-    // --- Agent credential writes ---
-    /// Write `github-token` into `AGENT_STATE_ROOT/<agent_name>/state/github-token`.
-    ///
-    /// The operator-supplied GitHub personal access token (PAT) for the
-    /// agent's GitHub integration (`services.hyperhive.agent.github.enable`).
-    /// hive-priv validates `agent_name`, creates the state dir
-    /// if absent, writes the file 0600, and chowns it to the agent so the
-    /// `gh` wrapper / git credential helper can read it. No account suffix
-    /// (single GitHub account per agent).
-    WriteAgentGithubToken {
-        /// Logical agent name (validated by `validate_agent_name`).
-        agent_name: String,
-        /// PAT value. hive-priv appends a trailing newline before writing.
-        token: String,
-    },
-
     /// Register the hive-ci Forgejo Actions runner: write the registration
     /// token to the host-side `/run/hive-ci/runner-token` env-file (root-owned,
     /// bind-mounted read-only into the container) as `TOKEN=<token>`, then