serve: add cwd flag and non-git root fallback

[stumct]

Why

When running opencode serve in containerized multi-repo workspaces (e.g. /work/src), the workspace root often is not a Git repo. In that case the server falls back to / as the worktree, which breaks permissions/config discovery and file ops.

What

• Add --cwd to opencode serve and pass it through as the server default directory.
• Treat an explicit --cwd as the root/worktree fallback when no Git root is found (so non-git roots behave like a real workspace).
• Allow JS SDK createOpencodeServer({ cwd }) to pass the flag through.

Notes

• Per-request directory query param / x-opencode-directory header still overrides the default.
• Git projects are unchanged.
• Related to PR feat(cli): add --cwd flag to web and serve commands anomalyco/opencode#7011: that approach updates serve/web to chdir, but does not change non-git worktree fallback behavior. This PR adds explicit non-git root/worktree handling when --cwd is provided.

Test Plan

• Not run (manual): opencode serve --cwd /work/src in a non-git workspace and confirm file ops/permissions resolve under /work/src.