core: a read-only task board draws no write and says nothing in place about why #2273

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment on this issue.

Written-by: FlyBot Lead
Claimed-by: FlyBot Lead
Claim-Id: c8d7d9828d3d46cd973dd6f1a50bc5e2
Branch: dev/1-p474-board-readonly-hint
Base: 5008116
At: 2026-11-03T05:39:13Z

In a read-only window, the task board draws no write at all, and nothing inside the board says why the writes are gone.

Reproduction on live main 50081167e81ac5ee182e6cc5d1a25ab248329e32 (a read-only window, i.e. the roster carries a non-empty readOnly sentence):

1. Open a Bot's task board. KanbanDialog reads the lock once (core/src/client/kanban.js, the locked value around :395) and the board correctly drops Add task, Start, Complete and draggable — that is the #2248/#2261 fix and it is not in question here.
2. Look for a sentence explaining the lock. There is none. Everything that explains read-only mode lives outside the overlay: core/src/client/settings.js draws bt-hint bt-read-only with the host's own sentence, core/src/client/sidebar.js draws the banner — and the board is a 1080px dialog that covers both.
3. So a member who cannot move a card sees a board with no controls and no reason. #2248 scoped exactly this out ("No new copy is invented on the board; the sidebar's read-only sentence is the explanation that already exists"); this issue is that follow-up, not a re-report.

Checked on that main: kanban.js already carries the .bt-board-hint class (:496 in its style block) but uses it only for the usage hint at :365; the file contains no hostText and no role: 'status' line today (0 occurrences each). So this needs no new copy and no new style.

Expected behaviour:

• A read-only board draws one line above the columns: h('p', { className: 'bt-board-hint bt-read-only', role: 'status' }, hostText(roster.readOnly)) — the same source and the same class Settings uses, so the text is the host's sentence, not a second wording.
• A writable board renders exactly what it renders today, with no such line.
• Nothing else moves: the read-only board still offers no Add task, no Start/Complete and no draggable card; a writable board still writes as before.

Scope:

• core/src/client/kanban.js — the one line in the board view.
• core/test/kanban.test.js — two assertions inside the existing P-455 case (no new fixture): read-only draws exactly one role=status line whose text equals hostText(roster.readOnly) and still no write; writable draws zero such lines.
• Regenerated only, never hand-edited: core/client.js, patches/ds-bot/0008-client-bundle.patch.
• changelog.d/ fragment (user-visible change), and whatever arch-docs requires for the touched path.
• Out of scope: the wording itself (that is the host's), the sidebar banner, and any other read-only surface.

Tier, by path: core/src/client/**, core/test/**, core/client.js, patches/ds-bot/** and the changelog are all T1, so this is T1. It regenerates core/client.js and patches/ds-bot/0008-client-bundle.patch, so it belongs to the same single bundle lane as #2246/#2265/#2264/#2268 and must not be opened against a moving base on its own.

Dedupe, checked before opening (live main 50081167 grep + open and closed issues): #2248 is closed and explicitly scoped this out; #2188 is closed (the banner disappearing when the sidebar collapses to the rail, a different surface); #2243, #2247, #2271, #2267 are other read-only gaps, none about a sentence inside the board. No open issue covers an in-board read-only explanation.