Enable readline by default for command-line history/editing/completion

USE_READLINE was a complete, working, opt-in-only feature -- input.c's
readline()/add_history() integration had no known reason to still be
gated behind a flag nobody was passing. Flips it to default-on
(renamed to a DISABLE_READLINE opt-out, matching the project's own
ifndef-DISABLE_* convention) and fixes the build: readline/history
live under the FreeBSD ports prefix (/usr/local), which this box's
clang/ld don't search by default, so linking bare -lreadline failed
until -L/usr/local/lib (LIBRARIES) and -I/usr/local/include (PROFILE)
were added alongside it.

Unlike the GENERATE_*_BITBOARD cleanup, kept DISABLE_READLINE as a
real escape hatch rather than removing the toggle outright -- this
introduces an actual external library dependency (readline-devel),
not just dead source-level conditionality, so a box without it
installed still needs a way to build.

Verified: both the new default build and `DISABLE_READLINE=1` build
clean (confirmed via ldd that only the latter has no libreadline.so
dependency), and a clean DEBUG=1 TEST=1 self-test run under the new
default with no assertion failures. Batch mode (used by all of this
project's scripted testing) never touches the input thread/readline
path at all, so this has no effect there.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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