Keys: one table file for the device's help panel and the website's key tables (#72)

Every screen's keys are constant tables in lib/core/src/app_keys.h (52 of
them, each under an `// id: Title` comment). An App's help() picks the table
of the state it is in. site/tools/gen_dev_docs.py reads the same file and
writes site/data/keys.toml; the `keys` shortcode shows a screen's tables on
its guide page, and /guide/keys/ shows all of them.

The Site job fails when the data file is out of date or a page asks for a
table that doesn't exist, and now also runs when app_keys.h changes. A key
added to an App shows up on the website without anyone editing a page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

site/content/guide/system.md

@@ -18,3 +18,9 @@ System changes nothing: it shows. It samples once a second and keeps its history
 ## Why it exists
 
 The device has about 107 KB of free memory and no PSRAM, so memory is the resource that decides what can run together: a secure connection takes about 52 KB at its peak. System makes that visible, and is how the project measures its own changes.
+
+## The keys, as the device lists them
+
+What <kbd>Fn</kbd> + <kbd>h</kbd> shows on these screens. These tables are generated from the firmware's own lists, so they are always the current ones.
+
+{{ keys(scopes=["system", "system-tasks", "system-system"]) }}