refactor: finish post and workspace naming cleanup

[benvinegar]

Summary

• rename internal flow/result terminology from surface artifact to post while preserving legacy wire aliases
• move CLI and stdio MCP internals toward canonical /api/posts endpoints
• replace user/agent-facing “board” terminology with “workspace” where it refers to sideshow workspaces
• rename workspace snapshot and Worker screenshot helpers to post/workspace terminology

Validation

• npm run typecheck
• npm test (414 passed)
• npm run lint
• npm run format:check

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