fix(chat): report the service tier a run resolved to

[devswha]

Refs #160.

The gap

The app pins the model, the reasoning effort and the model chain per run, and reports all three. It never named the service tier.

A measured app session recorded this at startup:

APP  service_tier_change    → "priority"
CLI  (no record)

while:

$ grep -rniE 'serviceTier|service_tier' server/ shared/ src/
(no functional match)

The app could not display, audit or explain the tier its own runs billed at, and the user's ~/.gjc/agent/config.yml does not set one either — so nothing on either side of the boundary could account for it.

Why it matters

The runtime's own schema is explicit about the cost meaning:

Processing priority hint (none = omit). OpenAI accepts the tier values directly; Anthropic realizes priority as speed: "fast" on supported Opus models.

So the tier changes what a turn costs, on the same model, at the same effort. Reporting the effort but not the tier tells half the story.

What this does

session.serviceTier was already a public getter, one call away, beside the thinkingLevel that gjc-session-state.ts has read since it was written. This:

• reads it into the same session snapshot (server/gjc-session-state.ts),
• validates it the same way as every other reported string on the client (src/contexts/sessionStatusSnapshot.ts, including the equality check that decides whether to re-render),
• renders it in the Agent sidebar's Environment block, which already documents itself as "reports, not controls" and already omits a row whose fact is unknown.

agentSidebar.environment.serviceTier added to all 10 locales; scripts/check-locale-parity.test.mjs passes.

Absent stays absent

undefined is the runtime saying "omit service_tier", which is its own default (the serviceTier schema default is none). No row is rendered rather than one claiming a tier the request never carried. Same for a blank or wrongly typed value.

Scope

This closes the visibility half of #160. Making the tier selectable needs a Settings surface and is deliberately not in this PR — #160 stays open for it.

Verification

node --test server/gjc-session-state.test.ts                          11 pass  0 fail
node --test src/contexts/sessionStatusSnapshot.test.ts                11 pass  0 fail
bun test  AgentSidebarEnvironment.dom.bun.test.tsx                     9 pass  0 fail
node --test scripts/check-locale-parity.test.mjs                       2 pass  0 fail
npm run check:identity                                                passed
npx tsc -p tsconfig.json --noEmit / -p server/tsconfig.json --noEmit  clean
npx eslint <changed files>                                            clean

New tests cover: the tier is read off the session; an omitted or blank tier stays absent; a throwing tier getter does not take the rest of the snapshot down (this runs inside the turn's event path); the client drops a wrongly typed tier; the tier participates in the snapshot equality check; and both render cases in the DOM.