docs(local): mark Orbit Local MCP page as roadmap — orbit mcp serve does not exist yet

What

Rewrites docs/source/local/access/mcp.md as a roadmap page. orbit mcp serve and glab orbit local mcp serve do not exist in any released Orbit CLI version (verified against v0.74.0, released June 9). The page previously carried a Beta status badge and config examples that fail silently.

Changes

• Added a Not yet available warning callout at the top, linking !1377 (the implementation MR) for tracking.
• Removed the details/history blocks claiming the MCP server shipped as an experiment in 19.0 and beta in 19.1. It never shipped.
• Aligned the Planned interface section with !1377's actual shape: run_sql, get_graph_schema, index, speaking raw DuckDB SQL over stdio. The old page contradicted itself (query_graph in the intro, run_sql in the table).
• Replaced the workaround examples with commands verified against v0.74.0: glab orbit local sql and bare glab orbit local schema. The query subcommand and the --ontology flag were removed from the binary.
• Marked inbound MCP mentions as planned in _index.md, getting-started.md, cli.md, and glab.md so no local page presents MCP as a working access path.

Review follow-up

• Duo flagged a definitions vs gl_definition table-name inconsistency: fixed, both examples now use gl_definition (verified against the binary).
• Duo flagged the glab.md MCP section presenting the command as working: !1604 (merged) (merged) reframed that section as planned. This MR additionally fixes its stale query_graph tool name.

Out of scope

v0.74.0 removed the query and compile subcommands and the schema --ontology/--query flags, which several local pages still document. Tracked separately (follow-up issue).

Validation

• mise run lint:docs: the only vale errors are pre-existing on main (CONTRIBUTING.md and two dev docs), identical count with and without this change.
• Workaround commands run against orbit v0.74.0 and a real local graph.

Closes #643 (closed)