feat: add worktreeOpencodeConfig for per-loop opencode config

[chriswritescode-dev]

Summary

Adds loop.worktreeOpencodeConfig — an inline opencode config that gets written as opencode.jsonc into each freshly created loop worktree, enabling per-loop customization (MCP servers, model overrides, etc.) without modifying the host config or polluting loop commits.

Changes

• worktree-opencode-config module — writes inline config to worktrees; respects existing committed opencode.json/opencode.jsonc (skip-if-exists); git-excludes the file so it never appears in loop commits
• forge-adapter integration — calls worktree config setup during worktree creation flow
• Config schema — adds loop.worktreeOpencodeConfig to forge-config.jsonc and PluginConfig types
• Documentation — documents the option in configuration.md and adds a per-loop customization note to loop-system.md
• Version bump — 0.5.0-beta.9 → 0.5.0

Files

• forge-config.jsonc — config schema addition
• src/index.ts — module wiring
• src/types.ts — type definitions
• src/version.ts — bump to 0.5.0
• src/workspace/forge-adapter.ts — integrate worktree config into creation flow
• src/workspace/worktree-opencode-config.ts — new module
• src/workspace/index.ts — export new module
• docs/configuration.md — document the option
• docs/loop-system.md — per-loop customization note
• test/workspace/forge-adapter.test.ts — adapter integration tests
• test/workspace/worktree-opencode-config.test.ts — new module tests