decouple plan view styling from nabat

[atkrad]

Summary

• internal/plan/view now owns its styling vocabulary: six roles (RoleTitle, RolePrimary, RoleDiffAdded, RoleDiffRemoved, RoleDiffModified, RoleDiffContext) and a one-method Styler interface. The package no longer imports Nabat or Charm.
• Options.Theme is replaced by an optional Options.Styler. Nil renders plain text, and WriteJSON ignores it.
• Diff intent is no longer mapped onto status colors. A deleted line is RoleDiffRemoved, not an error, and an update heading is RoleDiffModified, not a warning. The Nabat mapping belongs to the CLI adapter in a later PR.

Plain Human output is byte-identical to main. The old zero theme quietly replaced tabs with 4 spaces inside Lip Gloss, so the renderer now does that itself before calling the styler. A test pins this behavior.

Not in this PR: the deployah plan cutover, the Nabat adapter in internal/cmd/plan, and removal of the legacy planner. The legacy formatters under internal/plan still import nabat.dev/theme.

Test plan

• Unit tests added/updated
  • Role goldens (*.roles.golden, style_*.golden) record the role for every styled line. Each golden's text column must match the non-blank lines of the plain output.
  • A test checks that WriteHuman writes the string the styler returns.
  • A tab-expansion test covers both plain and styled output.
  • The existing Human and JSON goldens are unchanged.
• go build ./..., go test ./... -race, go vet, golangci-lint run, and pre-commit all pass
• go list -deps ./internal/plan/view lists no nabat.dev or Charm packages

Labels

• kind/chore
• skip-changelog

Checklist

• Title is short and imperative (matches commit style)
• No secrets or local-only paths in the diff

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/plan/view/human.go	95.12%	2 Missing ⚠️

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