docs: say that --issues picks a destination, not a run mode, in simplify and config

[atomantic]

Summary

Open PR #136 already covers most of #135 — it rewrote the --issues bullet and the --scan-only gate in better.md, better-swift.md, depfree.md, lib/plan-issue-mode.md, and lib/review-config-defaults.md. This is the disjoint remainder: the two places that still describe --issues as if it were a run mode, on files #136 does not touch (no overlap, no conflict).

• commands/do/simplify.md — the pass-through flag list lumped --issues/--issues-label in with the flags that stop the run (--scan-only, --no-merge), which is precisely the misreading --issues reads as a run mode but is a sink selector; --scan-only --issues is underspecified #135 was filed about; the issue's Context section names a /do:simplify --issues run as where it surfaced. --issues now gets its own bullet stating that it selects where deferred findings are recorded, that the run still remediates / PRs / merges, and that --scan-only --issues files every surviving finding as the run's entire output.
• commands/do/config.md — both places describing the saved issues default (the prose paragraph and the key reference) now note that issue mode changes only where findings are recorded, never whether the run remediates. This is the config key --issues reads as a run mode but is a sink selector; --scan-only --issues is underspecified #135 identifies as load-bearing: someone with {"defaults": {"issues": true}} needs to know it does not silently disable fixes.

review.md and rpr.md are deliberately untouched — both already scope their --issues bullet to deferred findings, the same reasoning #136 gave for leaving review.md alone. The --triage alias floated in #135 is also out, matching #136's decision that one discoverable spelling (--scan-only --issues) beats two.

Refs, not Closes, since #136 carries the Closes #135 trailer and lands the larger half.

Remaining

Nothing beyond what #136 already ships. Once both merge, #135's acceptance criteria are covered except the optional --triage alias, which both PRs explicitly declined.

Test plan

Docs-only; no executable behaviour changes.

• npm test — 187/187 pass
• git diff --stat confirms only simplify.md, config.md, and the changelog entry changed
• No file overlap with PR docs: say plainly that --issues picks a destination and --scan-only stops the pipeline #136 (better.md, better-swift.md, depfree.md, lib/plan-issue-mode.md, lib/review-config-defaults.md)

Refs #135