Add Zellij runtime adapter

[proboscis]

Summary

• Switch zellij adapter from session-per-run to tab-per-run model: each job now runs in a named tab inside a shared session, reducing session proliferation and aligning with zellij's tab-oriented design
• Promote session_is_running / session_exists from bool to Result<bool> so zellij CLI errors surface instead of being silently swallowed
• Add tab_exists() helper (via zellij action query-tab-names) for tab-level liveness checks in is_alive() and attach()
• Expose ZellijAdapter in runbox-core's public API (lib.rs re-export)
• Add zellij-tests feature flag in runbox-cli/Cargo.toml (mirrors existing tmux-tests pattern) to gate integration tests that require a live zellij binary
• Update CLI --runtime help text and attach command docs to reflect zellij parity with tmux
• Add feature-gated test_attach_zellij integration test covering the full spawn → tab-create → attach path

Acceptance Criteria

• Tab-based spawn: spawn() creates a background session, opens a named tab, runs the command with --close-on-exit, and verifies the tab exists before returning a RuntimeHandle
  • Evidence: zellij.rs diff — three-step session_output → tab_output → run_output sequence with tab verification
• Tab-level liveness: is_alive() checks both session running AND tab exists
  • Evidence: is_alive() now calls session_is_running(session).unwrap_or(false) && tab_exists(session, tab).unwrap_or(false)
• Error propagation: session_is_running / session_exists return Result<bool> with .context(...) on every zellij CLI call
  • Evidence: Both helpers use .output().context("...") and return Ok(...) / bail!(...)
• Public API: ZellijAdapter re-exported from runbox-core
  • Evidence: lib.rs diff adds ZellijAdapter to the pub use runtime:: line
• Feature gate: zellij-tests feature added to Cargo.toml; integration test gated with #[cfg(feature = "zellij-tests")]
  • Evidence: Cargo.toml diff + attach.rs test function annotation
• CLI parity: --runtime arg docs updated to list zellij alongside tmux; attach hint shown for both runtimes in cmd_run_replay
  • Evidence: main.rs diff — three --runtime help strings updated + matches!(runtime, RuntimeType::Tmux | RuntimeType::Zellij) guard
• Existing test assertions updated: test_attach_background_not_supported and test_attach_no_runtime_not_supported now expect "only supported for tmux/zellij"
  • Evidence: attach.rs diff lines 67 and 110
• Working tree clean after commit: git status shows nothing to commit, working tree clean
• Branch pushed: git push -u origin issue/ISSUE-031/run-20260308-105100 succeeded with new branch created on remote

Notes

⚠️ Backward-compatibility caveat: Any persisted RuntimeHandle::Zellij records created before this change store a full-UUID session name. After this deploy, is_alive / attach will fail to find those sessions (session name format changed from runbox-<full-uuid> to runbox-<8-char-short-id>). Existing in-flight runs should be stopped before upgrading.

Refs: ISSUE-031