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

frontend/packages/dashboard/build.mjs

@@ -1,5 +1,5 @@
 // esbuild build for @hive/dashboard: one JS+CSS bundle per page (H0M3 at
-// `/`, dashboard/flow/logs/core/builds/credentials at their own
+// `/`, dashboard/flow/logs/core/builds at their own
 // `.html`), each named after its own entry point below — see the
 // `entryPoints`/`for` lists for the exact map, not repeated here to
 // avoid this comment drifting out of sync with the real build steps.
@@ -40,7 +40,6 @@ await build({
     src("stats.js"),
     src("core.js"),
     src("builds.js"),
-    src("credentials.js"),
   ],
   outdir: staticDir(""),
   bundle: true,
@@ -107,7 +106,6 @@ for (const entry of [
   "stats.css",
   "core.css",
   "builds.css",
-  "credentials.css",
 ]) {
   await build({
     entryPoints: [src(entry)],
@@ -126,7 +124,6 @@ for (const html of [
   "stats.html",
   "core.html",
   "builds.html",
-  "credentials.html",
 ]) {
   copyFileSync(src(html), dist(html));
 }