refactor(tui): simplify components, drop dead code, consolidate helpers

[dgageot]

Two behavior-preserving refactor commits over pkg/tui/components/. Build, go vet, golangci-lint run ./... (0 issues) and the full go test ./... suite all pass.

Net diff: −277 / +103 across 17 files (one file deleted, two added).

Commits

1. refactor(tui): drop dead code and unused options across components

• scrollview: remove unused WithGapWidth option.
• sidebar: remove unused WithLayoutConfig, the Option type, and the opts ...Option parameter on New (no caller passes options).
• reasoningblock: remove the BlockMsg / blockMsgBase / ToggleMsg / GetBlockID infrastructure — no code ever constructs a ToggleMsg, so the matching switch case in messages.Update was unreachable.
• messages: drop the unreachable case reasoningblock.BlockMsg: branch and the orphaned forwardToReasoningBlock helper.
• tool/registry: remove the unused single-name Registry.Register(name, builder) overload (only RegisterAll was called).
• markdown: move NewGlamourRenderer into fast_renderer_test.go as newGlamourRenderer (only tests use it). Production renderer.go no longer imports glamour or styles.
• message: move stripANSI (and its regex) into message_test.go. Production message.go no longer imports regexp.

2. refactor(tui): simplify tool factory and consolidate render helpers

• tool: collapse Registry + Factory into a single static lookup table in factory.go; delete registry.go. The previous abstraction (Registry, Factory, Registration, NewRegistry, NewFactory, sync.RWMutex) was never instantiated externally and added indirection over a static map. tool.New API is unchanged. The map is initialized once at package load and only read afterwards, so the mutex is not needed.
• toolcommon: introduce NoArgsRenderer for tools that only show the icon + tool name (e.g. user_prompt, todo helpers). userprompt.go and todotool/component.go collapse to a one-line New().
• toolcommon: add Pluralize(count, singular, plural) and use it from listdirectory, directorytree, and searchfilescontent (each had its own copy/variant). The formatCount unit test moves to toolcommon/pluralize_test.go alongside the helper.
• toolcommon: drop the unused Base getters (Message, SessionState, Width, Height, Spinner) — every renderer already gets these via the Renderer signature.
• tool/factory.go: clearer package doc comment spelling out the lookup order.

What was deliberately not touched

• The big files (markdown/fast_renderer.go, messages/messages.go, editor/editor.go, editfile/render.go). Heavily exercised by tests; refactoring them is high-risk and orthogonal to readability.
• tab / tabbar (different concepts despite similar filenames; both used).
• The other public scrollview options (WithReserveScrollbarSpace, WithWheelStep, WithKeyMap) — all in active use.
• The uniform Renderer(msg, spinner, sessionState, width, height) signature, even where individual renderers ignore parameters, because Base orchestrates spinners and resize uniformly.

Validation

• go build ./... ✅
• go vet ./... ✅
• golangci-lint run ./... ✅ — 0 issues
• go test ./... -count=1 ✅ — all packages green
• deadcode -filter pkg/tui/components/ ./... ✅ — no remaining unreachable code

Internally reviewed (no bugs, security issues, concurrency issues, or coverage gaps found).