fix(runtime): emit contextRemaining in token_usage events so the TUI ctx segment renders

[Astro-Han]

Problem

The TUI statusline ctx segment (ctx 32k/128k 25%) never renders. The entire pipeline is wired but the source never produces data:

• TokenUsageEvent.contextRemaining is defined (packages/core/src/events.ts:487)
• ai-sdk-flow.ts:465-466 forwards it to StoredMessage
• pi-transcript.ts:478 stores it on state.usage
• pi-transcript.ts:946 gates the ctx segment on usage.contextRemaining !== undefined

But ai-sdk-backend.ts:1770–1790 constructs the TokenUsageEvent without ever setting contextRemaining. The field is always undefined, so the gate is always false, and the ctx segment is always omitted.

#1064 identified the symptom and #1066 addressed the chrome side (color thresholds, cwd shortening), but deliberately omitted a fallback using token_usage.input because that field is a billing-cumulative sum across tool-loop steps — using it as "used context" would produce misleading percentages (potentially >100%). The correct fix is to set contextRemaining at the source.

Root cause

ai-sdk-backend.ts has both pieces of data needed to compute contextRemaining:

1. contextWindow — from resolveSelectedModelContextWindow(connection, modelId) (already used for mid-turn capacity compaction at line 2558)
2. Last step's inputTokens — from the finish-step chunk's stepUsage.inputTokens (captured at line 1525 as stepUsage, accumulated into completedStepUsage)

The last step's inputTokens is the actual prompt token count of the final API request = how much context is in use. contextRemaining = contextWindow - lastStepInputTokens.

tokenUsage.inputTokens (the totalUsage used for the event's input field) is NOT suitable — it is cumulative across all tool-loop steps, which is why #1066 correctly rejected it as a fallback.

Desired outcome

• Capture the last step's inputTokens from the finish-step handler (not the cumulative totalUsage).
• When constructing the TokenUsageEvent, compute contextRemaining = max(0, contextWindow - lastStepInputTokens) when both values are available, and include it on the event.
• The TUI ctx segment renders without any further changes — all downstream plumbing already exists.
• Desktop's context display (if any) also benefits if it consumes the same field.

Non-goals

• Changing the fix(cli): statusline density, color fallback, and activity-strip spacing #1066 decision to omit a fallback using cumulative input — that decision is correct.
• Adding contextRemaining to providers that don't report per-step usage — this is an ai-sdk-backend fix only.