Use the slices package to simplify slice operations

[dgageot]

Replace manual loops and append-slice tricks with their slices/strings standard-library equivalents, and tidy a few small helpers along the way. Pure refactor — no behaviour change, all tests still pass.

slices package usage

File	Before	After
pkg/hooks/builtins/add_directory_listing.go	sort.Strings(names)	slices.Sort(names)
pkg/fsx/collect_test.go	sort.Strings(got)	slices.Sort(got)
pkg/tui/components/messages/messages.go	append(s[:i], s[i+1:]...) (×2)	slices.Delete(s, i, i+1)
pkg/tui/components/editor/editor.go	append(e.attachments[:i], e.attachments[i+1:]...)	slices.Delete(...)
pkg/tui/service/supervisor/supervisor.go	manual index loops + append-delete + append-insert	slices.Index / slices.Delete / slices.Insert
pkg/tui/dialog/working_dir_picker.go	manual index loop in cycleSection	slices.Index
pkg/config/hooks.go	12× append(append([]T{}, base...), cli...)	12× slices.Concat(base, cli)
pkg/hooks/handler.go	append([]string{}, base...)	slices.Clone(base)
pkg/runtime/runtime_test.go	append([]tools.Tool{}, toolList...)	slices.Clone(toolList)
pkg/session/migrations_unit_test.go	append([]Migration{}, migrations...)	slices.Clone(migrations)

Small tidy-ups

• pkg/config/hooks.go: extracted defsFromCommands and matcherFromCommands helpers, collapsing five copy-pasted blocks in HooksFromCLI into a single struct literal.
• pkg/hooks/handler.go: in hookEnv, replaced the manual for ... if ch == '=' scan with strings.Cut, and dropped the redundant index update on the append branch.
• pkg/tui/service/supervisor/supervisor.go: removed the noop if runner.NeedsAttn { runner.NeedsAttn = false }, and dropped the unnecessary named return on CloseSession.

go build ./..., go vet ./..., golangci-lint run ./... (0 issues) and go test ./... all pass.