Lint round 5+6: staticcheck and errcheck to zero (#2130)

* refine the plan

* Fix all staticcheck lint findings (uncapped)

golangci-lint's default max-same-issues=3 was hiding most of the backlog:
the true pre-fix count was 69 staticcheck findings, not 34. This fixes all
of them, driving staticcheck to zero:

- ST1023/QF1011 (37): omit explicit types inferred from the RHS
- S1009/S1031 (15): drop redundant nil checks before len()/range
- SA9003 (9): remove comment-only empty branches, keeping the comments
- QF1007 (3): merge conditional assignment into declaration
- QF1006 (3): lift break conditions into loop conditions
- QF1001 (3): apply De Morgan's law / name the negated predicate

Also updates plans/lintfixes.md with the cap discovery and the corrected
errcheck picture (1202 uncapped, ~949 of them fmt.Fprint*).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Drive errcheck to zero: config for bulk categories, propagate real errors

Adds .golangci.yml with errcheck exclude-functions for fmt.Fprint* (usage
printers), (*bufio.Writer).Write/WriteString (sticky errors, surfaced at the
now-checked final Flush), and (*strings.Builder).WriteString; pins
max-issues-per-linter/max-same-issues to 0 so CI reports true counts.

Real error paths now propagate instead of being dropped:
- Finalize{Reader,Writer}Options in join/put/filter/split/tee and the
  repl/script entry points: 'mlr join -i badformat' now errors instead of
  silently using wrong separators
- final output-stream Flush in pkg/stream: write failure no longer exits 0
- DSL emit/print/dump redirect writes, matching their sibling branches
- CSV writer WriteCSVRecordMaybeColorized, close-time Flush in file output
  handlers, ENV[...] Setenv, REPL record-write and redirect-close errors
- termcvt write-side Close before rename (had "TODO: check return status")

The rest are deliberate ignores, marked with _ = and a comment where the
reason isn't obvious: unset-of-missing-path no-ops, read-side closes,
mid-stream FlushOnEveryRecord, init-time strftime registrations, in-memory
usage-capture pipes, and regtest-harness env/temp-file teardown.

golangci-lint now reports 0 issues on ./cmd/mlr ./pkg/... with all caps off.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

pkg/stream/stream.go

@@ -101,7 +101,9 @@ func Stream(
 		}
 	}
 
-	bufferedOutputStream.Flush()
+	if err := bufferedOutputStream.Flush(); err != nil && retval == nil {
+		retval = err
+	}
 
 	return retval
 }