[Fix #31] feat(upgrade): one-shot v1->v2 upgrade via bundled skill + command

[diegohb]

Summary

• Ships a bundled skills/opencode-v2-upgrade/SKILL.md and commands/upgrade-opencode-v2.md, loaded via new SkillLoader/CommandLoader and registered from the plugin through ctx.skill.transform and ctx.command.transform.
• /upgrade-opencode-v2 re-submits its template through ctx.session.prompt with the skill attached, giving a one-shot v1->v2 upgrade flow.
• Extracts a shared FrontmatterParser used by the agent, skill, and command loaders (removes duplicated parse logic); registration reads as a single Effect.gen sequence.
• Drops the inert agent binding from the command frontmatter and makes the skill's facts-record contract self-contained.
• Documents the flow and its safety contract in README, AGENTS.md, the CONTEXT.md glossary, architect routing/reference notes, and oneshot Example 10.

Why

• v1 extension packages need a safe, guided path to v2 (C8: One-shot consumer upgrade capability — skill + command #31).
• The skill enforces a verifiable safety contract: upgraded output must validate against the pinned v2 schema and carry no v1 remnants.

Testing

• New golden v2 fixture (tests/fixtures/v2-extension-package/) with tests proving the upgraded config/plugin/agent/skill and report validate against the pinned schema and leave no v1 remnants.
• Harness now audits skill and command registration; loader, fact-gate, and fixture-coverage tests added (tests/bundled-content.test.ts, tests/load-hook.test.ts, tests/v2-host.test.ts).
• Existing v1 extension package fixture retained as upgrade-source coverage.