docs: fix build/install paths and add explicit agent responsibilities

[randomm]

Task Description

AGENTS.md references install.sh at the wrong path (packages/opencode/script/install.sh — should be ./script/install.sh from repo root). Additionally, the build and install section lacks explicit guidance on which agent handles which step, causing confusion and failed installations.

What needs fixing

1. Wrong path: AGENTS.md line 332-333 references packages/opencode/script/install.sh but the script is at repo root ./script/install.sh
2. Missing agent responsibilities: No guidance on who does what (ops builds, but cp/codesign requires different permissions)
3. install.sh installs to ~/bin/opencode-new: AGENTS.md does not mention this candidate/stable binary pattern or the promotion step
4. Build is run from packages/opencode, install from repo root: The directory navigation between steps is unclear

Quality Gates (Non-Negotiable)

• Documentation: AGENTS.md accurately reflects actual file locations and workflow
• Fork-features: Update agents-config manifest entry if content changes

Acceptance Criteria

• AGENTS.md Build and Install section has correct path to install.sh (./script/install.sh from repo root)
• Explicit step-by-step with working directory shown for each command
• Agent responsibilities documented (who builds, who installs, who verifies)
• Candidate binary pattern (opencode-new then promote to opencode) documented
• Fork-features manifest updated if AGENTS.md content changed