Style/cpplint fixes

[vjan-nie]

style: apply cpplint fixes, filter what conflicts with 42/C++98

Second phase of the team style adoption (after the clang-format PR).
Runs cpplint (Google style) and resolves every finding under an explicit
priority order: 42's subject > C++98 > project decisions > Google style.
Google is an adopted baseline of good practices, overridden wherever a
higher layer conflicts.

Adopted (applied to the code)

• runtime/explicit — Bot(Server*) marked explicit; prevents accidental
  implicit conversion of a Server* into a Bot.
• readability/braces — three if/else made brace-symmetric; prevents the
  "second statement silently outside the else" class of bug.
• include_what_you_use — FancyLogSink.hpp now includes <string>
  directly instead of relying on a transitive include.
• build/include_order — 8 files regrouped into Google's ordering
  (own header, C system, C++ system, project), which also verifies each
  header is self-contained. All compiled clean → headers were already
  autonomous.

Filtered (documented in CPPLINT.cfg, each a higher-authority conflict)

• legal/copyright — 42 projects don't carry copyright headers.
• build/include_subdir — a Google-monorepo include-path convention, N/A
  to this tree.
• build/header_guard — Google wants path-based guards
  (INCLUDE_CLIENT_HPP_); 42's CPP modules expect header/source name parity
  and short guards, and there are no cross-directory name collisions, so the
  path convention solves a problem we don't have.
• runtime/references — passing by const reference is idiomatic C++;
  Google's pointer-for-mutables rule is a call-site house preference.
• runtime/int — fixed-width types aren't guaranteed in strict C++98;
  the subject uses int/size_t/time_t.

Verification

cpplint clean (exit 0), three tiers -Werror clean, full suite 511/511, no
regression. Config lives in CPPLINT.cfg (cpplint 1.6.1 reads that, not
.cpplintrc) with the rationale inline.

Responds to issue #22