usage: convert .txt+generator to a plain .cc raw-string literal

usage: convert .txt+generator to a plain .cc raw-string literal

The previous approach generated bees-usage.c from bees-usage.txt via a
make rule using sed.  That added a generated C file to the build, which
did not survive 'git clean' without first re-running make.  It also
made the usage text invisible to grep and git log -p without escaping.

Replace bees-usage.txt + the make generator rule with a plain .cc file
containing a single raw-string literal assigned to BEES_USAGE.  The
file follows the same pattern as bees-config-v1.cc and compiles via the
existing %.cc pattern rule in src/Makefile without special treatment.

No behaviour change: the string stored in BEES_USAGE is identical to
the previous generated output.  The --help output is byte-identical.

Assisted-by: Claude-Code:claude-sonnet-4-6
Signed-off-by: Zygo Blaxell <bees@furryterror.org>