feat: add support for other AI platforms other than Claude Code

Description

Epic for the open-standard / multi-harness effort.

Strategy: publish skills to the open agent-skills standard (.agents/skills/ convergence + skills.sh registry, https://agentskills.io/home) instead of maintaining per-harness plugin manifests. Tiers:

• Tier 1 — Claude Code: full plugin (skills + agents + recommend-plugins hook + MCP). Primary, always supported.
• Tier 2 (wave 1) — Codex + Gemini CLI + OpenCode: skills verified + CI-tested. All three read .agents/skills/ directly, support MCP (context7), and support subagents. Codex and Gemini ship Claude-compatible hook systems (JSON-over-stdin), so the recommend-plugins hook ports with adaptation; OpenCode uses a JS/TS plugin API, so its hook is a rewrite.
• Experimental: every other harness that reads .agents/skills/ (Cursor, Antigravity, Copilot, ...) — skills work for free, no dedicated CI or guarantees.

Subagents port to tier 2

Wingspan's power is its subagents (analysis / codebase-review / quality-review / research agents, and the review/plan panels). Good news: both tier-2 harnesses now ship full subagent support with isolated context, curated tools, MCP, and parallel dispatch — so these flows port rather than degrade.

• Gemini CLI (subagents since v0.36): custom agents are Markdown + YAML frontmatter in .gemini/agents — nearly identical to our existing agents/*.md, so close to a copy.
• Codex (subagents GA March 2026): custom agents are TOML in ~/.codex/agents/, driven by spawn_agent / wait_agent primitives and /agent. Requires an md -> TOML conversion of our agent definitions.
• OpenCode: markdown subagents (mode: subagent, invoked via @mention) — close to our existing agents/*.md.

Remaining work is format conversion, not reimplementation of the workflow.

Linked under this epic

• #224 release in skills.sh
• #225 support AGENTS standard
• #226 Codex support (tier 2)
• #227 Gemini CLI support (tier 2)
• #229 OpenCode support (tier 2)
• #228 Phase 0 portability prep — flatten namespacing + cross-plugin collision audit ($ARGUMENTS / ${CLAUDE_SKILL_DIR} fallbacks partly landed via #220), context7 MCP guard, reserved-command audit, agent-definition format conversion (md -> TOML for Codex; md+frontmatter for Gemini; md agents for OpenCode) (blocks the rest)

Related

• PR #220 (GitHub Copilot CLI dual-manifest) — pending decision: merge-and-freeze as the last per-harness manifest, or close in favor of open-standard-only. Its $ARGUMENTS / ${CLAUDE_SKILL_DIR} fallbacks and host-aware hook are reusable regardless.
• Closed prior attempts: #16, #22 (Cursor).

Requirements

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Additional Context

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