docs(linkedin): document the mentionable-people directory endpoint + MCP tool

[sergebulaev]

Documents the LinkedIn mentionable-people directory shipped in publora/publora.com#218 (closes publora/publora.com#215).

What changes

• New endpoint page docs/endpoints/linkedin-mentionables.md — GET /linkedin-mentionables (q/limit query params, response field table, curl + JS examples, error table incl. 403 UPGRADE_REQUIRED, "How the Directory Fills" section, cross-links to the mentions guide).
• Mentions guide — new "Who Can I Mention?" section (connected accounts + engagement-captured people), recommends the endpoint / linkedin_list_mentionables MCP tool as the discovery path, plus a new bullet under Finding LinkedIn URNs and a Related link.
• MCP docs — linkedin_list_mentionables section in docs/mcp/tools-reference.md; tool listed in docs/guides/mcp-server.md; active-tool count bumped 14 → 15 in tools-reference.md and client-setup.md.
• Indexes — endpoint rows added to README.md (table + LinkedIn bullets) and SKILL.md.
• OpenAPI — added the /linkedin-mentionables path to schema/openapi.yaml (validated: file parses, Error schema already carries code).
• Changelog — additive entry dated 2026-07-17.

How to verify

• python3 -c "import yaml; yaml.safe_load(open('schema/openapi.yaml'))" — passes.
• Facts (params, response shape, 403 body, fill mechanism, paid-plan gating) match the merged implementation in publora/publora.com#218.

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