[Tioit-Wang]

Adds a plugin-side channel for one short suggestion in the Composer, so a
plugin can propose the user's next message where the user is already looking,
without a toast, a notification, a bubble, or a panel.

What a plugin gets

pi.composer.suggest({
  sessionId,     // the session from session:turnEnded
  turnId?,       // the turn the text answers; an overtaken turn is refused
  text,          // one line; whitespace folded, cut at 200 chars
  ttlMs?,        // default 120000, clamped to 1000..600000
}): Promise<{ shown: true } | {
  shown: false;
  reason: "session-not-visible" | "draft-not-empty" | "stale-turn" | "replaced" | "unsupported";
}>

pi.composer.clearSuggestion({ sessionId? }): Promise<{ ok: true }>

Permission: composer.suggest, risk medium (confirm at install). It can
put text in front of a user who may accept it with one key, but it cannot read
the draft, cannot send, and cannot outlive an empty draft — the same trust shape
as composer.transform.

A place that will not show the line is not an error: the call resolves
{ shown: false, reason } so a plugin can log the miss. Only malformed input
throws (INVALID_PARAMS), and a missing grant throws PERMISSION_DENIED.

How the decision is split

• Main refuses what it already knows: a sessionId that is not the session
  on screen, and a turnId that onTurnStarted has since overtaken. It then
  relays the offer and waits (2 s) for the renderer's answer.
• Renderer refuses the rest — no composer, a blocked composer, a draft with
  text or an attachment — and answers over pluginComposerSuggestionResult, so
  the plugin learns the real outcome.

Main never writes renderer state; the offer is a message, exactly like
pi.ui.revealView.

Lifecycle

Shown only while: the named session is on screen, the draft is empty (no text,
no attachment), the named turn is still the session's newest, and a composer
takes input. One suggestion per session; a newer offer replaces the older one.

Cleared by: any draft change (typing, pasting, an attachment,
composer.insertText, composer.replaceDraft), a send, a session switch, a new
turn starting, the plugin unloading/being disabled, clearSuggestion, a
renderer reload, a window close, and the suggestion's own TTL.

Keys: Tab accepts (draft = text, caret at end, input focused, staged
attachment kept), Esc dismisses, and the @ / / completion list keeps Tab
and the first Escape.

Visual: the existing .composer-placeholder layer, brighter (62% primary ink),
one line with an ellipsis, plus a localized Tab / Esc hint and one polite
sr-only announcement. aria-placeholder is unchanged.

Compatibility

Additive: new permission, new SDK namespace, two new IPC channels, one i18n key.
A plugin that does not declare composer.suggest behaves exactly as before. No
manifest schema version, host-core schema, or persisted-format change.

Verification

• apps/desktop/test/composer-suggestion-state.test.mjs — renderer state
  machine (empty draft, session match, attachments, accept/dismiss, typing,
  session switch, TTL expiry, host clears, key arbitration with completions).
• apps/desktop/test/composer-suggestion-relay.test.mjs — main relay (session
  visibility, stale turn, replacement, timeout, turn start, plugin stop, quit).
• apps/desktop/test/plugin-composer-suggestion.test.mjs — real plugin process
  calling pi.composer.suggest / clearSuggestion: text shaping, TTL clamp,
  scope refusal, PERMISSION_DENIED without a grant, INVALID_PARAMS.
• apps/desktop typecheck; packages/{shared,plugin-sdk,i18n} builds and
  vitest suites; plugin-runtime-risk (permission matrix parity),
  main-wiring-contract, composer/plugin suites; biome lint; style-token guard.

See docs/adr/0332-plugin-composer-suggestion.md.