feat: forward skills, agent roles, MCP notes and OpenCode-only instructions

Since the catalog fix in 1.3.3 Claude only received the Code Mode
catalog from OpenCode's system prompt. That dropped the skills list,
which the skill tool needs for its ids, custom agent prompts such as a
writer subagent's rules, MCP server notes, and OpenCode-only instruction
files.

Forward each of those as its own block, found by its own markers rather
than by section order: the skills list, the custom agent's prompt (text
before OpenCode's model identity, skipped for OpenCode's stock prompts)
labelled as the session's role, the <mcp_instructions> block, and
instruction files OpenCode loaded that Claude Code doesn't read itself.
A project's AGENTS.md is left to Claude Code, which loads it where there
is no CLAUDE.md, and files Claude Code already reads as CLAUDE.md are
skipped. The runtime note drops the model and effort, which Claude
Code's own prompt already names, and the tool note names the host app.

Tested with unit cases for each block and live through opencode run: a
writer agent's rules, the skills list without tools, no role for build,
and an AGENTS.md-only project read once, by Claude Code.