[opencode] Sprint 3: Command spec trim + version pinning + SKILL.md cleanup

[SkyWalker2506]

Sprint 3 Tasks

• S3-T1: Split commands/opencode.md into smaller files: opencode-zen.md, opencode-local.md, opencode-models.md
• S3-T2: Update main opencode.md to delegate to subcommand files
• S3-T3: Add minimum version check in opencode-status.sh (opencode --version >= required)
• S3-T4: Reduce SKILL.md trigger set — remove overly broad triggers like individual model names
• S3-T5: Update README with corrected alias behavior and config file location

Depends on: #3 (Sprint 2)
Priority: P2 medium