OMNIS-34: Add the plan preview and apply handoff to the TUI

[JtheGunner]

YouTrack: https://youtrack.xn--wrm-hoa.li/issue/OMNIS-34 (sub-task of OMNIS-14)

In omnishell tui, the key a shows the plan that omnishell apply would execute, in a scrollable screen. Confirming it closes the UI and runs the real omnishell apply; apply's own Proceed? prompt stays the gate, so the preview is not a confirmation. No new dependencies.

Changes

• engine: new Engine.Preview(cfg, lockPath) returns the rendered plan (plus a warning per unknown module) and NeedsApply. NeedsApply uses apply's own rule, plan.HasChanges || initOrRCDrift, so the preview never promises a change that apply would not make (a merely deleted init file is deliberately not drift to a plain apply). It is read-only.
• tui: Backend gains Plan(). The model opens a waiting screen at once and computes the plan in a tea.Cmd (Update still does no I/O). Esc cancels; an answer for a request the user already left, or for an older request, is dropped. a is ignored while a toggle is being written. The plan screen scrolls (arrows, j/k, page up/down, home/end, g/G), and shows only "back" for a failed plan or a plan with nothing to apply. Plan text and errors go through sanitize.
• tui: Run returns a Result; y/Enter on a shown plan sets Result.ApplyRequested and quits.
• cli: after the UI has closed and the terminal is back to normal, omnishell tui runs the apply command with every flag at its default (no --yes) and returns its error, so prompts, sudo, hooks and exit codes are apply's own. A failing UI never leads to an apply.
• README, CHANGELOG and CLAUDE.md describe the behaviour; the implementation plan is under docs/superpowers. go.mod and go.sum are unchanged.

Commits

• docs: add implementation plan for the plan preview in the TUI (OMNIS-34)
• feat: add Engine.Preview to show what apply would do
• feat: let the tui backend compute the plan
• feat: add the plan screen to the tui model
• feat: draw the plan screen in the tui
• feat: hand over to apply when the plan is confirmed in the tui
• docs: describe the plan preview in the omnishell tui

Test plan

• make vet and make test (race detector) pass locally.
• golangci-lint v2.13.2 (the CI version, via go run) reports 0 issues.
• CGO_ENABLED=0 go build ./cmd/omnishell succeeds for linux/amd64, linux/arm64, linux/armv6, linux/armv7, darwin/amd64 and darwin/arm64.
• TestPreviewNeedsApplyAgreesWithApply compares the preview with a real Apply for a fresh config, a converged one, a deleted init file and a newly enabled module.
• The handoff was run with the real binary in a pseudo-terminal (throwaway HOME): plan, y, then n at Proceed? exits 1 with "aborted" and writes nothing; plan, Esc, q exits 0; plan, y, then y runs apply and writes init.zsh. An independent review ran it as well.
• CI: vet, race tests, golangci-lint, govulncheck and the build matrix pass.
• Reviewer: with a scratch HOME (never a real config), run omnishell init, omnishell enable completion, then omnishell tui. Press a and check that the plan appears and scrolls, Esc returns to the same selection, and y followed by n at the prompt changes nothing. Then again with y at the prompt.

Known gaps (not in this PR)

• Pressing Enter after y answers apply's Proceed? [y/N] with No (safe, but it looks like a failure); y/Enter also hand over when the terminal is currently too small to show the plan; an a, Esc, a, Esc burst leaves read-only package-manager probes running; very long plan lines and errors are cut without an indication.
• If apply finds nothing to do after the handoff (the config changed since the preview), the UI closes without any output.
• Existing behaviour on main: apply echoes the package manager's brew info output to the terminal while it computes its plan. OMNIS-35 only quieted the read-only commands.
• The options editor is the next sub-task (OMNIS-33).