chore: collect release notes for v0.1.0-alpha.4

[loganthomas]

Folds the staged news fragments into CHANGELOG.md under the v0.1.0-alpha.4 section and deletes them. The release workflow will publish this section as the GitHub release notes when the tag is pushed.

Enhancements

• Pool mode is here: wt init provisions a pool of pre-warmed slot-N worktrees, wt new/wt done claim and release them with crash-safe leases, wt claim/wt release expose the plumbing, and wt pool ls/wt pool resize manage the pool; separately, wt init now pre-fills its answers in any repo by scanning the root, where a tracked lockfile proposes a hash-gated refresh hook and untracked .env files propose a copy list. (Phase 4: pool mode — leases, claim/release, slot pool #11)

Documentation

• New docs/pool-mode.md explains slots, the claim/release loop, leases, and the refresh gate, new docs/recipes.md explains how caches are shared across trees, with per-ecosystem setups (Bazel, npm/pnpm/yarn, Go, direnv, ports), and docs/configuration.md gains a "Detected defaults" reference for what wt init proposes from a repo-root scan. (Phase 4: pool mode — leases, claim/release, slot pool #11)