feat: add opencode skill install support

[jtdelia]

Adds dci skill opencode subcommand, which installs the DCI skill to ~/.config/opencode/skills/dci-cli/.

OpenCode discovers skills by directory convention using SKILL.md as the entry point — consistent with Claude, Kiro, and Gemini. No agent-specific config file needed.

[Copilot]

Pull request overview

Adds OpenCode as a supported target for the existing dci skill installer flow, enabling installation into OpenCode’s expected config directory layout.

Changes:

• Add opencode to the supported agent list for dci skill, targeting ~/.config/opencode/.

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[Copilot]

The supported agent list is now updated to include opencode, but the test suite still hard-codes the agent list in TestInstallSkill (main_test.go) without opencode. This makes it easy for future additions to drift and leaves this new support unverified. Consider adding opencode to the test cases (and/or deriving the test list from the same source as the CLI command list) so the new install target is covered.

[apgiorgi]

Thanks for the contribution, John!

[apgiorgi]

Two issues to address before merging.

main_test.go — TestInstallSkill not updated: The agents slice in TestInstallSkill mirrors main.go and must be kept in sync. Add {"opencode", ".config/opencode"} to that slice (after the {"gemini", ".gemini"} entry, before the closing } at line 1079) to cover the new entry.

[apgiorgi]

OpenCode is the only entry using an XDG config path (.config/opencode) instead of a dotfile. Per project convention, add a brief inline comment explaining why — e.g.:

{"opencode", ".config/opencode"}, // OpenCode uses XDG config dir (~/.config), not a dotfile