fix: block sends whose text and attachments exceed the per-message limit

[TheDarkPhantom]

Problem

Each attached document passed its own 500k-character cap, but a turn merges the typed text and every document into one message capped at 600k. Nothing checked that total until ProviderSendTurnInput decoded it in ProviderService.sendTurn, after the message was already persisted. The thread kept a turn the agent never saw, plus a "Provider turn start failed" row that dumped the entire prompt.

Fix

• Shared rule (packages/shared/src/documentContext.ts): appendDocumentContexts (the join the Normalizer already did) and turnInputTooLargeReason, so server and composer measure the same thing and say the same sentence.
• Server (Normalizer.ts): thread.turn.start / thread.fork measure text plus document blocks before any upload is claimed and fail the dispatch with the plain reason. Nothing is persisted, and pending uploads stay for a retry.
• Headroom (contracts/provider.ts): the late provider check now allows the referenced-thread context (≤100k) and style directive (≤2k) on top of the 600k user budget, so a message accepted at dispatch cannot fail there.
• Composer (ChatComposer.tsx): over the limit, Send (button and Enter) is disabled and a red line under the attachments explains why. Removing an attachment clears it.

Verification

• vp test run on documentContext.test.ts, Normalizer.attachments.test.ts (new: two 350k docs rejected, nothing claimed, uploads kept), Normalizer.test.ts, parseChangelog.test.ts, and apps/web/src/components/chat: 443 passed.
• tsgo --noEmit for contracts, shared, server, web; vp lint clean on touched files.
• Integrated pass in the dev desktop renderer: two 350k text files show "This message and its attachments come to 700,966 characters; the limit is 600,000…", Send disabled, Enter does nothing; removing one clears the note and re-enables Send.

Claude Opus 5.5 (1M context) in Claude Code

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