Plans screen: design features not built (paste import, editing, proposals, warnings, questions, approval)

Problem

docs/plan-format.md ("How a plan gets into codeboost") and the design doc describe six Plans screen features that were not built:

importing a plan by pasting it (the screen accepts a .json, .yaml or .yml file only: web/public/index.html, docs/implementation/planning-screen.md);
editing the plan on the Plans screen, with each change checked as you type and approval saving the revision.
showing a runner's plan-change proposal (task in needs amendment) on the Plans screen so a person can approve or edit it (design doc, "When the plan is wrong", step 4). The runner does not save such a proposal yet either (steps 1–2; today a needs-amendment pause comes from an out-of-scope edit), so this needs lane F runner work first.
showing check warnings on plan items (docs/plan-format.md, "Checks after import");
answering open questions from the screen; it lists them read-only today (docs/plan-format.md, questions row).
a whole-plan approval step ("approve anyway" after warnings or open questions); today an import or an applied draft or suggestion creates the next revision directly (docs/plan-format.md).

T18 did not require any of them, and Lane G (#145) closed without them. PR #154 marks them as not built in docs/plan-format.md and the design doc, and links here.

Acceptance

Decide whether each feature is still wanted. If not, remove it from docs/plan-format.md.
If wanted: the proposal view shows the saved proposal and its partial changes read-only; paste import goes through the existing POST /api/plan/import with the same revision binding, action-ID replay and size limit as file import (see Align plan import request limit with valid schema payloads #146); in-screen editing produces the next revision only through a Store-owned, revision-bound endpoint, and keeps unsent Review drafts.
Browser tests in test/browser/plans*.spec.ts cover each feature that is built.
As each feature is built or dropped, update every note that cites Plans screen: design features not built (paste import, editing, proposals, warnings, questions, approval) #155: docs/plan-format.md ("What the shipped Plans screen does today"), docs/implementation/planning-screen.md (Handoff), and docs/designs/codeboost-plan-indexed-review.md (the summary's "Where the build is", the note after "When the plan is wrong", build step 6, "What to do next", and the DT status list).