A read_log tool and six MCP resources over the log and the catalogue

read_log fetches the live log over FTP and filters by tag, severity and
message before it returns anything, because half an hour of uptime is
five thousand entries. It excludes shp_usbmode by default, a five-second
USB poll that can be a third of the log, and says in the result that it
did so. Only the current file is reachable: the archives the drone
rotates into are a previous owner's sessions and carry network and
location data, which is a decision for a person, not a tool.

bebop://commands is the resource worth having, because it needs no
drone: the whole catalogue is readable with the aircraft switched off.
The ones that do need a link return an explanation rather than raising,
since a resource that errors looks broken while one that explains itself
is empty for a reason. bebop://state also reports telemetry age, because
the aircraft accepts a second controller and silently redirects
telemetry to it, so a connected flag proves nothing.

The log says nothing resolvable about ARSDK, measured rather than
assumed: no command ids appear anywhere in it, and the COMMANDS function
names match arsdk-xml for 4 of 11 symbols, 3.5% of the tag. So nothing
here resolves them, and read_log says where to look instead.

_host moves into tools/_common as drone_host now that three tool modules
need it, and head/tail in the shell allow-list gains the reason they
must take a file argument: ld.so.preload makes SIGPIPE fatal, so a pipe
into head writes a crash report to the aircraft's flash.

tests/test_tools.py

@@ -32,7 +32,7 @@ async def test_every_tool_is_registered(client):
     assert names == {
         "connect", "disconnect", "connection_status", "list_commands", "command_info",
         "send_command", "get_state", "watch_state", "preflight_check", "camera_snapshot",
-        "camera_record", "list_files", "fetch_file", "shell_read", "arm", "disarm",
+        "camera_record", "list_files", "fetch_file", "shell_read", "read_log", "arm", "disarm",
     }  # fmt: skip