feat(third_party): add PostHog MCP plugin (posthog-mcp)

[minupalaniappan]

What this adds

Re-lists the URL-only PostHog connector (removed in #436) as posthog-mcp / "PostHog MCP". Same content as #431: mcp.json → https://mcp.posthog.com/mcp, README, changelog, MIT license, PostHog favicon logo. Linear PGT-4565.

Why a new name

PostHog publishes its own APPROVED plugin as posthog (id 730, from PostHog/ai-plugin, with bundled skills/commands/hooks). Two APPROVED plugins sharing a name collide in the indexer (the Coinbase stale-draft failure), so this one ships under posthog-mcp. The README states it is the URL-only connector and that users should install one or the other, not both.

Auth verification

Unchanged from #431: anonymous POST /mcp → 401 with resource_metadata; AS oauth.posthog.com with PKCE S256 + DCR; DCR returned 201 for every redirect set Cursor and Grok send, including Grok Bot mobile and grok.com.

What to review

• The MCP server key inside mcp.json stays posthog (that is the name clients show for the connection); only the plugin name/dir/display name changed. Flag if you want the server key renamed too.
• Real review question: do we want both PostHog cards visible in Grok Bot, or should this one carry Grok-Bot-only gating (cursor: "never") so Cursor users keep seeing only the vendor plugin?
• node scripts/validate-plugins.mjs passes for all plugins.

Out of scope

grok.com catalog connector; hosted SERVER placement check after publish; account-based sign-in test (draft until then).

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Note

Low Risk
Marketplace and connector metadata only; no changes to core plugin runtime or auth beyond pointing clients at PostHog’s hosted endpoint.

Overview
Adds a new marketplace plugin posthog-mcp / PostHog MCP under third_party/posthog-mcp, re-listing the URL-only connector to PostHog’s hosted MCP at https://mcp.posthog.com/mcp (OAuth; no bundled skills/commands).

Registers the plugin in .cursor-plugin/marketplace.json with metadata, mcp.json, README (capabilities and install notes), changelog (name chosen to avoid indexer collision with PostHog’s vendor posthog plugin), and MIT license. The MCP server key in config remains posthog while the plugin id is posthog-mcp.

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