iso-harness v0.5.0

Merge — not overwrite — shared config files with iso-route

@razroo/iso-route writes model routing config to .codex/config.toml (model, [profiles.*], [model_providers.*]) and opencode.json (model, agent.*). Previously iso-harness's Codex and OpenCode emitters wrote those same files from scratch, stomping everything iso-route had put there. The @razroo/iso wrapper runs iso-route first and iso-harness second, so the net effect was that Codex and OpenCode users lost all model routing config on every composed build.

Codex: the emitter now reads any existing .codex/config.toml, strips only the [mcp_servers.*] sections (iso-harness's domain), preserves everything else, and appends the freshly-rendered MCP block.

OpenCode: the emitter now reads any existing opencode.json, preserves every field except $schema and mcp, and layers its own $schema + mcp + user-declared extras on top.

Behavior when no prior file exists, or when running iso-harness standalone without iso-route, is unchanged.