docs(opencode): fix build/install paths and add agent responsibilities (#396)

[randomm]

Fixes #396

Changes

• Corrected install.sh path (was packages/opencode/script/install.sh, should be ./script/install.sh from repo root)
• Added agent responsibilities table (ops builds/installs, user promotes)
• Documented candidate binary pattern (opencode-new → promote to opencode)
• Added step-by-step with working directories for each command
• Updated fork-features manifest with new criticalCode markers

Quality Gates

• Typecheck passes (docs-only change)
• Fork-features verify passes (249 tests)
• Adversarial review: APPROVED