feat(sessions): list and resume OpenCode sessions from the Resume Session picker #773

Motivation

With an OpenCode pane focused, the Resume Session picker silently lists Claude sessions and resuming one launches Claude: savedSessionListing is false for opencode (src/providers/shared/featureCapabilities.ts:141), listSessions is async () => [] (src/providers/registry.main.ts:104), and CommandPalette.tsx:431-436 falls back to DEFAULT_PROVIDER. The registry comment claiming no such list exists is stale: opencode session list --format json -n N (run in the project cwd) returns [{id, title, updated, created, projectId, directory}] (verified on 1.18.27), which maps onto SessionInfo.

Intended behaviour

• listOpencodeSessions(cwd, limit) next to exportOpencodeSession in opencodeCliSessions.ts (id→sessionId, title→summary, updated→lastModified, fileSize 0), wired as listSessions in the registry; savedSessionListing: true.
• The picker spawns with providerRuntime: 'terminal' when the focused pane is terminal-flavoured; "Resume Session" description no longer says Claude/Codex only (tabCommands.ts:65).

Acceptance criteria

• Focusing an OpenCode pane and opening Resume Session lists that project's OpenCode sessions and resumes the chosen one in a terminal-runtime pane.
• opencodeCliSessions.test parses the verified JSON shape; providerFeatures.test.ts updated.

Refs #739 (smart resume), #772.