docs(v3): refresh docs for daemon-first architecture

[Codename-11]

Summary

• Adds four new v3 user-facing docs: docs/architecture.md, docs/daemon.md, docs/protocol.md, and docs/v2-to-v3-migration.md. Each links back to docs/plans/arc-v3-daemon.md as the source of truth.
• Rewrites the Getting Started "Run ARC" section around arc daemon start, ARC_PORT, and ARC_DIR. Adds a v2 deprecation notice at the bottom and updates the ToC (docs/index.md) with the new v3 section.
• Adds a 1.0.0-alpha.0 entry to CHANGELOG.md in Keep-a-Changelog format (Added / Changed / Breaking / Removed) covering the daemon skeleton, binary-mux WS protocol, client SDK, relay placeholder, and arc daemon CLI group.
• Adds a "v3 in progress" callout and lightly refreshes the opening section of the README.md — no wholesale rewrite.

Wire-protocol content (frame diagram, envelope schema, method catalog, schemas, topics, error codes, backward-compat rules) was hand-derived from packages/client/src/protocol.ts.

Test plan

• pnpm install (clean)
• npx tsc --noEmit (clean — docs-only change)
• pnpm site:build (clean)
• pnpm build:docs (clean — VitePress build of user-docs/)
• Every internal link in new docs resolves (docs/plans/arc-v3-daemon.md, cross-refs between the four new docs, package source links).
• Every code snippet cross-checked against live source (packages/daemon/src/, packages/client/src/protocol.ts, packages/cli/src/commands/daemon.ts).
• Protocol claims that diverged from the current implementation (Zod parse errors silently drop vs. returning bad_request; auto-probe for daemon health) softened to match reality.

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