inputSchema is scoped to marketplace listings only, undocumented as such, with no general input-introspection path

Split from #1869 (credit: @subheeksh5599).

**Problem**
`workflows.inputSchema` is a real column ("v1.7 listing fields") populated (and required) only for public MCP-listed workflows; it's null by design for private/draft workflows, never derived from node `data.config`. `docs/api/workflows.md` never states this scoping. The one documented dynamic-schema path (`docs/agent/mcp-trigger-inputs.md`) exists per listed workflow only - it doesn't help an agent introspecting its own non-listed workflow, the default first-integration case.

**Open question**
Populate `inputSchema` (or equivalent) generally, or document it as listing-only and design a separate general introspection mechanism for non-listed workflows?

Start with docs/api/workflows.md and docs/agent/mcp-trigger-inputs.md, then trace how workflows.inputSchema is scoped to public MCP-listed workflows. Done requires resolving and documenting the intended scope, or defining the separate general introspection path for private and draft workflows.

Written by the indexing model from the issue text.