Rewrite manage-apis skill from verified platform behavior, #6

[edipsopa]

Closes #6

Full rewrite of the manage-apis skill based on the current MCP server implementation in planet9, with every claim verified against the source and live-tested on an internal DXP instance (clearly-named probe artifacts, all deleted afterwards).

Headline changes vs the previous skill:

• apiType is settable at creation (the old "read-only over MCP" section was stale) and is now documented as never changed after creation — save_api won't stop a flip (verified live: it persists silently and produces an unsupported artifact), so the skill hard-forbids it and gives a recreate-and-migrate path instead.
• Correct recipe for script-backed APIs, including request/response definitions that survive MCP's silent field stripping: property.reference, parent, isParent, and parameter.type are dropped without any error (verified live) — the root cause of the reported "couldn't fix the request/response types" failure. The skill prescribes the shapes that provably store and export correctly: flat definitions, content-level references, inline nesting only in content[].items.
• Partial-update semantics (verified live): absent key = preserved, null array = wiped, present array = replaced wholesale — so minimal payloads are the safe default and a get→save round-trip of imported definitions is a destructive operation the skill warns against.
• Role assignment over MCP is a silent no-op (the save schema accepts role but the entity relation is roles; verified live) — routed to the Cockpit; the previous role: write recipe never worked.
• Traps documented: upsert-on-unknown-id creates ghost records, duplicate names are allowed, delete_api reports success for nonexistent ids, no endpoint protocol validation, and Script Editor hides MCP-created APIs (enableScriptEditor is not settable over MCP).
• Corrected runtime semantics: restrictAccess applies only when the role check doesn't run; enableProxy is a consumer-side switch (App Designer / generated-client routing), not a proxy-route gate; table-API runtime access is gated by the table's own roles, not the API artifact's.
• New "What MCP can NOT do" section and an error/symptom table.

A detailed verification log (each claim mapped to a source location or a live experiment) is available and can be attached as a PR comment on request.

Also noticed while verifying: the 24.15 docs page mcp-server/apis.html still has an intro paragraph saying APIs are exposed read-only over MCP, while the same page documents save_api/delete_api — worth a docs fix.