fix: enforce context token budget

[Kokeip]

Summary

• enforce a real prompt token budget from context-window and output-reserve settings
• compact message history semantically, prune oversized tool results, and retry once after provider context overflow
• anchor estimates to provider-reported prompt usage and expose estimate/compaction details in the Web UI
• normalize context-window overflow errors across OpenAI-compatible, Anthropic, Bedrock, and Google adapters

Scope

This PR contains only context accounting and compaction. The world-topology feature remains separate and is not included.

Validation

• pnpm --filter @cantilune/boot test — 51 files, 534 tests passed
• pnpm --filter @cantilune/adapter test — 18 files, 152 tests passed
• boot and adapter production TypeScript builds passed
• website server typecheck and streaming smoke test passed
• ESLint on context-touched files and git diff --check passed

Note: the full website client typecheck is currently blocked in the local mixed worktree by uncommitted topology work; those files are excluded from this branch and PR.