fix(opencode): use commands/ directory (plural) to match OpenCode docs

[marcusburghardt]

Description

Align the OpenCode integration's command directory with OpenCode's
documented standard: .opencode/commands/ (plural) instead of
.opencode/command/ (singular).

OpenCode's documentation (https://opencode.ai/docs/commands/) uses
commands/ as the canonical directory name. The OpenCode runtime
supports both via a {command,commands} glob, so the singular form
works but is outdated. OpenSpec already fixed this in
Fission-AI/OpenSpec#748.

This is a two-value change in opencode/__init__.py:

• commands_subdir: "command" → "commands"
• registrar_config.dir: ".opencode/command" → ".opencode/commands"

With corresponding test updates in test_integration_opencode.py
and test_integration_subcommand.py.

Backward compatibility

No user action is required. OpenCode loads commands from both
.opencode/command/ and .opencode/commands/ via its
{command,commands} glob, so existing projects continue
working without changes. After updating, new specify init
runs will deploy Speckit commands to .opencode/commands/
(plural). Previously deployed Speckit commands in
.opencode/command/ remain functional but can be removed:

rm .opencode/command/speckit.*.md

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Ran specify init /tmp/speckit-test --integration opencode --script sh
and verified commands are created in .opencode/commands/ (plural)
with no .opencode/command/ (singular) directory created.

Also ran uv run python -m pytest tests/test_agent_config_consistency.py
(24/24 pass).

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

This fix was identified through codebase analysis using OpenCode
(claude-opus-4-6). The two-line source change and test updates
were authored with AI assistance. All tests were run and verified
locally.