dotcli: add dot setup folders (XDG short-name migration)

Adds a dot setup subcommand family (idempotent, re-runnable machine-setup
tasks) with a folders task that brings the 8 standard XDG user
directories under a fixed short-name convention (Desktop -> .desktop,
Documents -> doc, Downloads -> dwn, Music -> mus, Pictures -> pic,
Videos -> vid, Templates/Public -> .ignoreme), hardcoded rather than read
from ~/.config/user-dirs.dirs -- that file is a separate, manually
tracked dotfile whose values can drift or go stale, and the migration
must not depend on it being correct. The nested Pictures/Screenshots
folder is renamed to pic/screenshots in the same pass.

Content left behind in a legacy full-named folder by a fresh XDG-defaults
install -- empty or not -- is merged into its short-named replacement.
A same-named collision between a legacy folder and its target is never
overwritten: it is skipped, reported, and the legacy folder is left in
place rather than deleted while something in it could not be merged.
--dry-run previews what would move/skip without touching the filesystem.
xdg-user-dirs-update runs once afterward to notify running apps/portals.

.config/dot/.claude/tasks/000b-folders-unconditional-merge.md

@@ -0,0 +1,79 @@
+---
+spec: dot-setup-folders
+blocked-by: 0008-folders-collision-handling
+---
+
+## What to build
+
+Remove the `--yes` confirmation gate that 0007/0008 built: a legacy folder
+with real content in it is migrated unconditionally now, the same as an
+empty one, since the collision handling from 0008 already makes the merge
+non-destructive on its own (a same-named entry is never overwritten, and the
+legacy folder is kept whenever any collision occurred). The `--yes` gate
+turned out to protect against a scenario collision handling already
+prevents, while making the everyday case — a machine that already has real
+files in `~/Documents`, `~/Pictures`, etc. — a silent no-op unless the flag
+was remembered, which defeats the point of the task.
+
+In its place:
+
+- `dot setup folders` always attempts the merge for every legacy folder,
+  content or none.
+- A new `--dry-run` flag replaces `--yes` in the flag slot: it reports what
+  would move and what would be skipped as a collision, without touching the
+  filesystem at all (no `mkdir`, no `mv`/`rmdir`, no `xdg-user-dirs-update`).
+- A real (non-dry-run) run now reports what it moved per legacy folder
+  (e.g. `moved 12 entries from ~/Documents to ~/doc`), instead of staying
+  silent on success. A folder where nothing top-level moved (already empty,
+  or everything in it collided) prints no such line — only non-trivial moves
+  and collisions produce output.
+- `--yes` is removed outright (not kept as a silent no-op): passing it now
+  fails with argparse's standard unknown-option error.
+
+## Acceptance criteria
+
+- [x] A legacy folder with real content merges on a plain `dot setup
+      folders`, with no flag required
+- [x] A real run prints `moved N entries from ~/<legacy> to ~/<target>` for a
+      folder where top-level entries actually moved, and nothing for a
+      folder where none did
+- [x] A real run prints a dedicated line when the nested Screenshots folder
+      itself is moved (e.g. `moved ~/Pictures/Screenshots to ~/pic/screenshots`)
+- [x] Collision detection/reporting and the "leave the legacy folder in
+      place when a collision occurred" behavior from 0008 are unchanged
+      under the new unconditional default
+- [x] `--dry-run` reports the same would-move/would-skip information without
+      creating any target directory, moving/removing anything, or invoking
+      `xdg-user-dirs-update`
+- [x] `dot setup folders --yes` fails with an unknown-option error (argparse
+      default), rather than being silently accepted or gated on
+- [x] `dot setup folders help` output no longer mentions `--yes` and
+      documents `--dry-run` instead
+- [x] Idempotency holds: re-running after a clean merge, and re-running
+      after a collision was reported, both behave the same as before
+- [x] `~/.config/dot/tests/dot.fish` is updated to exercise the above
+      (replacing the old `--yes`-gated cases) and
+      `fishtape ~/.config/dot/tests/dot.fish` passes
+
+## Implementation Notes
+
+- The `--yes` gate and the `screenshots_emptyish`/`other_entries` machinery
+  that computed it were deleted outright rather than special-cased away:
+  once merging is unconditional, that machinery had no remaining purpose
+  (it existed solely to decide "empty enough to skip the gate").
+- `mkdir -p $target_path` and the final `xdg-user-dirs-update` are both now
+  guarded by `not set -q _flag_dry_run`, making `--dry-run` a true no-op
+  rather than "no-op except for directory scaffolding."
+- Collision detection (`test -e $target_path/...`) runs identically in both
+  modes; `--dry-run` only gates the actual `mv`/`rmdir`/`mkdir` calls, so the
+  reported would-move/would-skip split is exactly what a real run would do.
+- Success reporting is per-legacy-folder and suppressed at zero: a folder
+  that was already empty (or whose only entries all collided) prints
+  nothing, so a routine re-run stays quiet like before.
+- All prior collision/idempotency/Screenshots test scenarios were kept,
+  just re-pointed at the plain `dot setup folders` invocation instead of
+  `--yes`; two scenarios that only differed by which code branch (`--yes`
+  vs. silent-empty) they exercised now hit the same branch, but were both
+  kept since they still cover distinct fixture shapes (Pictures with vs.
+  without unrelated top-level content alongside a colliding Screenshots).
+- `fishtape ~/.config/dot/tests/dot.fish` passes (178 tests).