Extract `@domscribe/protocol` package (RCP versioning unit)

[Narrator]

Sprint 2371 — task T1 (Batch A)

Implements decision in RFC 0001: extract the agent-facing contract into a new @domscribe/protocol package, versioned independently from implementation packages, so RCP v1.0.0 can be published with a meaningful SemVer stamp.

Scope

• New workspace package @domscribe/protocol under packages/
• Relocate Zod schemas: ManifestEntry, RuntimeContext, Annotation, error envelopes (currently in @domscribe/core)
• Relocate MCP tool + prompt name constants
• Update relay, runtime, mcp to depend on @domscribe/protocol for shapes and names
• @domscribe/core keeps utilities (PII redaction, ID generation, error system)
• Sync nx project graph; update TECHNICAL_SPEC.md §2.3 module boundaries

Acceptance

• packages/protocol/ exists with its own package.json, builds independently via nx build protocol
• @domscribe/core no longer exports the relocated schemas
• All consumers (relay, runtime, mcp, plugins) import from @domscribe/protocol
• pnpm run build:all passes
• No new public API in @domscribe/protocol beyond the relocated surface (no scope creep)
• PR description references RFC 0001

Blocks

T2 (tool-name normalization lands in this package), T4 (spec doc references this package).

Out of scope

• FrameworkAdapter SDK extraction — separate follow-on RFC per RFC 0001 explicit deferral
• Wire-format changes — pure relocation, no shape edits this sprint