docs(langgraph): voice pass on guides + concepts

[blove]

Summary

Voice pass (1 of 7 per-lib PRs) applying Brian's technical register to the langgraph docs beyond getting-started. Prose only — no code, command, API name, type, version, link, heading, MDX component, or <Step title> changed.

• guides ×9 (lifecycle, streaming, subgraphs, time-travel, persistence, deployment, memory, interrupts, testing): contractions, "Let's" lead-ins, trimmed corporate filler, flagged opinions+tradeoffs where the page already recommended something (throttle default, server-side interrupt timeout, same-origin proxy).
• concepts ×5 (agent-contract, langgraph-basics, angular-signals, state-management, agent-architecture): warmed explanatory prose into the technical register; flagged the architecture/state-design stances as opinions with tradeoffs.
• api ×4: reviewed, already terse/in-voice — no changes (YAGNI).

Test Plan

• Heading/fence/<Step>/note guard clean; headings byte-identical to origin/main (rehype-slug anchors + TOC preserved).
• Independent voice review: GUARDRAILS PASS + VOICE PASS (prose-only; technical register; opinions tied to existing recommendations).
• All 18 langgraph routes return HTTP 200.

🤖 Generated with Claude Code

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