Proposal: a session-visible notice seam for plugin advisories

[Big-Dao]

A plugin that discovers an environment problem at session start has no way to
tell the person looking at the session. The available channels in
0.2.1-rc.2 are ctx.logger.warn (host log, invisible unless opened), a
runtime-context entry (model-visible, not a notification), and agent/error
(reserved for failures). There is no ctx.notices/toast service and no client
slot for one.

Concrete case: this repository's WSL plugin runs a session-start preflight for
bubblewrap, systemd PID 1, the user manager, systemd-run and Linger. A
failure is advisory — it must not fail the session or install anything — but it
is exactly the information the user needs before the first command fails
closed. Today the plugin publishes the advisory to the log and to a
session-scoped runtime-context entry; the GUI cannot render it as a banner.

Proposal: a small service plus a client slot, mirroring the approval service's
session-scoped contract:

interface SessionNotice {
  readonly id: string              // producer-owned; re-notifying replaces
  readonly level: 'info' | 'warning'
  readonly text: string
  readonly remedy?: string         // copy-paste fix
}
interface NoticeService {
  notify(session: Session, notice: SessionNotice): Promise<void>
  clear(session: Session, id: string): Promise<void>
}

Requirements: visible in the session before the first failed command;
session-scoped; severity + remedy; replaceable/dismissable (a fixed environment
must not keep the old warning); non-fatal (no turn cancelled, no tool result
changed). The host keeps the last value per (session, id) for reconnect and
replay.

Nothing here is model-facing: producers that also want the model to see the
advisory keep their runtime-context entry. The seam is presentational.

Full context, current workaround and the plugin's use of it:
docs/UPSTREAM-SESSION-NOTICE.md in
Big-Dao/dsh-plugin-wsl-env.