[dbdefault]

Closes #49

Adds an adapter for OpenCode (opencode run), tested against opencode 1.18.34 (recorded in the module docstring). Follows PR #27 (Droid/Devin).

What changed

• src/secfoo/agents/opencode.py: new OpenCodeAdapter (build_command, extract_report, extract_usage)
• Registered in registry.py and the AgentId enum in cli.py (secfoo run --agent opencode); -m model flag added to the model-selection test table
• README / docs/REFERENCE.md supported-agent lists
• base.py: one small optional hook, extra_env (class attribute, empty by default). env is passed to Popen only when non-empty, so existing adapters are unaffected.
• tests/test_agents/test_opencode.py: fake-binary tests, no real API calls

Command

opencode run --pure --format json --dir <workdir> [-m provider/model], prompt on stdin.

Requirements from the issue

• Non-interactive: prompt via stdin, no trust/auth prompt (verified: without credentials it exits with a 401 API error event instead of hanging).
• Read-only: --auto is not used. Because opencode's default agent allows bash/edit even without --auto, the adapter also sets OPENCODE_PERMISSION and OPENCODE_CONFIG_CONTENT to deny bash, edit and webfetch. Both are set so an inherited OPENCODE_PERMISSION or a project opencode.json that allows them cannot widen access. Manually verified with opencode debug agent build, and with a live run (free model) asked to create a file and run a shell command: both were refused and no file was created.
• Tests: fake binary, assert no --auto, assert both deny env vars reach the subprocess, assert other adapters still launch without an env kwarg.

Verification

• pytest tests/test_agents: 124 passed
• Real run against a free OpenCode model confirmed the JSON events the parser reads (text, step_finish tokens/cost).

Known limits

• Full pytest tests on my Windows machine (after secfoo vendor mermaid and installing the PDF/DOCX deps): 1002 passed, 4 failed, all unrelated to this change and Windows-specific: 3 POSIX file-permission-mode assertions (0600 is not representable on Windows) and 1 symlink test (needs symlink privilege).
• ruff check . reports pre-existing violations on main; the new files add none.
• I did not verify that per-agent permission overrides in a user's own opencode config are also blocked; the global and inherited cases are.