feat(opencode): add terminal runtime and transcript switching

[Juliusolsson05]

Summary

• add OpenCode Terminal as a separate new-agent choice while keeping OpenCode as the existing structured HTTP/SSE runtime
• run the native TUI in Agent Code's managed PTY lifecycle, preserving skills, scoped built-in MCP, durable identity, recovery, Dispatch, linked panes, undo, and duplication
• add OpenCode CLI export/import hosting for rewind, duplicate, compaction handoff, and all six Claude/Codex/OpenCode provider-switch directions
• pin the OpenCode decoder/projector from agent-transcript-parser PR fix(codex): scope resume picker to current cwd #23

Safety decisions

• keep terminal as runtime metadata, not a duplicate provider kind
• keep MCP bearer values out of argv and inline JSON through OpenCode environment interpolation
• use supported OpenCode import/export rather than its private SQLite schema
• refuse transcript transforms while an exported OpenCode turn lacks durable assistant completion

Verification

• root contracts, live-fixture check, keybinding check, typecheck, and live-resume probe passed
• focused OpenCode/runtime/switch/recovery suite: 15 files, 97 tests passed
• MCP policy/host suite: 4 files, 22 tests passed
• parser npm run check: 24 files, 100 tests, package verification passed
• isolated OpenCode 1.18.27 import/export round trips passed for blank and two-message sessions using temporary XDG state
• production application build and packaged-entry verification passed
• complete root run: 2,498/2,501 passed; two unrelated fixed-timeout tests pass independently (17/17), and one developer-local provenance fixture references a deleted personal Claude transcript (clean CI does not have that corpus)

Dependency: Juliusolsson05/agent-transcript-parser#23

Closes #754