GNOME parity for the session keepalive feature #12

Feature request: GNOME parity for the session-keepalive feature

Follow-up to #10 / #11, which added an opt-in "keep the Claude 5-hour session window open" feature to the KDE widget only, with GNOME parity explicitly deferred.

Scope

shared/core/keepalive.js (from #11) is already platform-agnostic — the decision logic and the buildShellCommand() PATH-prefixing need no changes. What GNOME needs is its own thin platform layer, mirroring how extension/lib/runtime/fetch.js and fs.js already wrap Soup and Gio.File in Promises for the shared core to consume:

• A new extension/lib/runtime/process.js wrapping Gio.Subprocess in a Promise, resolving {exitCode, stderr} — the same shape shared/core/keepalive.js already expects from KDE's ExecReader.qml.
• Two new keys in extension/schemas/org.gnome.shell.extensions.brainusage.gschema.xml: keepalive-enabled (boolean, default false) and keepalive-command (string, default the same verified ping command from Keep the Claude 5-hour session window open (opt-in) #10).
• Wiring the keepalive controller into the scheduler in extension.js's enable(), reading both settings live off the existing Gio.Settings object.
• No new preferences UI: neither this extension's one existing setting (panel-label-mode) has a graphical prefs page, so the two new keys follow the same convention (gsettings set / dconf-editor), documented in the README.
• The Claude-card warning line already renders through the shared buildUsageViewModel(), so no GNOME-specific UI work is needed there.

Why a separate issue/PR

shared/core/keepalive.js only exists on the feat/session-keepalive branch (#11) so far, not on main. This work is cut from that branch rather than from main, and the PR targets feat/session-keepalive until #11 merges.