fix: address gadfly swarm review of the executus re-platform

Folds in the real findings the dogfood swarm raised on PR #20 (21 graded
real, 1 false positive):

- tools.go: anchor get_diff `path` matching to whole path tokens (foo.go no
  longer pulls in barfoo.go; a trailing "/" still scopes a directory); split
  the diff once + cache it; drop the spurious trailing blank line; fix the
  "truncated after line N" off-by-one wording. Share one tool-list source
  (allTools) between toolbox() and the executus registry.
- executus.go: drop the dead Config.Defaults caps (per-run RunnableAgent always
  overrides them); shared envBool/reviewTimeout helpers; resolveContextTokens
  logs a failed lookup and uses a 5s timeout (was 15s); note the budget guard
  is pass-granular (the wall-clock backstop covers mid-pass).
- main.go/recheck.go: shared envBool; fix package-doc drift (the removed
  finalization fallback, the paginated get_diff).
- entrypoint.sh/run.sh: export GADFLY_MAX_DIFF_CHARS directly (run.sh prefers
  it); guard the watchdog's delayed SIGKILL on a .disarmed marker so it can't
  catch the consolidation pass.
- tests: anchoring test; corrected obsolete env var + truncation-wording asserts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/run.sh

@@ -177,7 +177,7 @@ case "$PROVIDER" in
           GADFLY_SYSTEM_FILE="${SCRIPT_DIR}/system-prompt.txt" \
           GADFLY_TITLE="$TITLE" \
           GADFLY_BODY="$BODY" \
-          GADFLY_MAX_DIFF_CHARS="$MAX_DIFF_CHARS" \
+          GADFLY_MAX_DIFF_CHARS="${GADFLY_MAX_DIFF_CHARS:-$MAX_DIFF_CHARS}" \
           GADFLY_STATUS_FILE="${GADFLY_STATUS_FILE:-}" \
           GADFLY_FINDINGS_OUT="${GADFLY_FINDINGS_OUT:-}" \
           "$BIN" 2>"$ERR_FILE"