docs(roadmap): fix stale Areas index and dead plugin-api anchor - #1095

Fixes #1088.

PR #890 dropped the landed per-area rows (#78/#82/#81) and deleted the ## Plugin API section, but missed the Areas index table at the top of ROADMAP.md. This PR tidies only that table:

• Remove the Plugin API row — its #plugin-api anchor is dead (the section was deleted by docs(roadmap): program tending 2026-10-29 — drop landed rows #890).
• Nested subsessions Current focus: drop "One follow-up left" (that follow-up was the landed feat(tui): nest the opencode mini Select subagent menu — follow-up PR #64 left behind #78 row) → "Shipped across the TUI, the app and ACP".
• mcpx integration Current focus: "Where the flake gets mcpx" (landed nix: the fork's flake builds opencode without mcpx unless the consumer injects one #82 intent) → "Tool projection and daemon lifecycle", matching the section prose.
• Serve handover Current focus: "Prove the handover under real systemd" (landed serve handover: verify under a real systemd user session and close the teardown race #81 intent) → "Session state outside the server process", the one remaining item in that section.

Verification: scripted anchor check over ROADMAP.md — every (#...) link (6 remaining) resolves to a heading in the file; dead anchors: none. Docs-only change; no other ROADMAP content touched.

Queue task 4589 (repo-health-opencode-pilot).

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