Check that documented tool flags exist

The remaining class of claim check_docs.py could not see: whether the commands in the
docs would actually run. A renamed or removed option is the classic form of command
rot, and the one that wastes a reader's time most directly -- they paste the line and
it fails. All 9 documented flags across screenshot.py, webui.py and restore_flash.sh
are real.

The check earned its own lesson, recorded in both languages. Its first version matched
only to the end of the line, so on a wrapped command it saw --frame-addr and nothing
after the backslash: 4 of 9 flags, and it reported a clean run. A check that silently
covers a quarter of what it claims is worse than no check, because the clean result is
believed. Continuations are joined before matching now.

Confirmed it fails when it should: renaming --frame-addr to something no tool accepts
produces two named failures and exit 1, and reverting returns it to clean.

check_docs.py now runs seven checks.

AGENTS.md

@@ -501,8 +501,19 @@ modules. All four were found by comparing against the source, none by proofreadi
 So the comparison is mechanical now. It checks that every tool a README names exists,
 that every test in `run_tests.sh` is documented in both languages, that internal `.md`
 links resolve, that the translation pairs have matching heading structure, that the
-memory-map addresses match the model's `#define`s, and that documented firmware
-`file:line` references still point at what the prose claims.
+memory-map addresses match the model's `#define`s, that every long flag a doc passes to
+a tool actually exists in it, and that documented firmware `file:line` references still
+point at what the prose claims.
+
+The flag check earned its own lesson. Its first version matched only to the end of the
+line, so on a wrapped command like
+
+    python3 tools/screenshot.py --frame-addr 0x200013DC \
+        --status-addr 0x2000175C --port 1234 --out screen.png
+
+it saw `--frame-addr` and nothing else -- 4 of 9 flags, and it reported a clean run.
+**A check that silently covers a quarter of what it claims is worse than no check**,
+because the clean result is believed. Continuations are joined before matching now.
 
 One caution, from writing it. An early version compared firmware constants with a regex
 that took the first number on the line, so `key_debounce_10ms = 20 / 10` read as 20 and