docs: add opencode-session-cost to plugins (#280)

Co-authored-by: igorvelho <[email protected]>

data/plugins/opencode-session-cost.yaml

@@ -0,0 +1,18 @@
+name: OpenCode Session Cost
+repo: https://github.com/igorvelho/opencode-session-cost
+tagline: Sidebar block showing true session cost including all subagents
+description: |
+  Subagent costs hide from you by default in OpenCode's session cost display.
+  This TUI plugin adds a sidebar block showing your true total — the active
+  session plus every subagent (task) it spawned — and lets you expand it to
+  see the cost of each one individually. Works for every session regardless
+  of model or provider, no configuration required.
+scope:
+  - global
+tags:
+  - cost
+  - session
+  - subagent
+  - tui
+  - sidebar
+min_version: 1.18.0