feat: add conversation and plan-step coordination primitives

[G9Pedro]

Summary

• add first-class conversation and plan-step built-in primitives, plus a new kernel conversation domain module for thread coordination, message/event timelines, plan-step lifecycle transitions, progress updates, and derived conversation state summaries
• extend core context graph contract/invariants to include conversation + plan-step primitives and relationships, and update kernel exports/snapshots accordingly
• add a dedicated CLI command module with conversation and plan-step command families (create/show/list/message/state/thread-add/thread-remove and create/list/start/progress/block/done/cancel), wired through existing runCommand JSON envelope
• add regression coverage in kernel conversation tests and CLI compatibility smoke tests for the new command surface

Test plan

• pnpm exec vitest run --config vitest.config.ts packages/kernel/src/conversation.test.ts packages/kernel/src/context-graph-contract.test.ts packages/kernel/src/registry.test.ts tests/integration/cli-compat.test.ts
• pnpm run typecheck
• pnpm exec vitest run --config vitest.config.ts packages/kernel/src tests/integration/cli-compat.test.ts
• pnpm run build
• pnpm run test (known unrelated failure: architecture test enforcing root src/ absence in repository state)

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