feat: open all sessions in new tabs; sidebar terminal links

Sidebar SessionItem was only opening web tool URLs in new tabs.
Terminal sessions linked to the project page in the same tab.
SessionCard 'Open' buttons for terminal sessions navigated in-place.

Changes:
- app-shell.tsx: SessionItem now builds terminal URLs
  (/instances/:id/terminal) and always uses target=_blank
- session-card.tsx: compute openHref for both web and terminal sessions,
  render <a> links with target=_blank instead of callback buttons
- use-instance-actions.ts: handleOpen now uses window.open(..., '_blank')
  for terminal sessions and project fallback

All session opening (sidebar, cards, callbacks) now consistently opens
in a new tab.

Quality gates: tsc clean

.atl/.skill-registry.cache.json

@@ -1,3 +1,3 @@
 {
-  "fingerprint": "fdea8a74bb4c7449c01c4bd61646c895b10ede78"
+  "fingerprint": "c36b11ec5edebc02aa51b1113a7a11dc2559e812"
 }

.atl/skill-registry.md

@@ -2,7 +2,7 @@
 
 <!-- Auto-generated by gentle-pi extensions/skill-registry.ts. Run /skill-registry:refresh to regenerate. -->
 
-Last updated: 2026-05-28
+Last updated: 2026-06-02
 
 ## Sources scanned
 
@@ -21,7 +21,6 @@ Last updated: 2026-05-28
 | Skill | Trigger / description | Scope | Path |
 | --- | --- | --- | --- |
 | `auto-commit` | Use when you are making multiple edits or completing significant work in a git repository to automatically create commits | user | `/home/alex/.config/opencode/skills/auto-commit/SKILL.md` |
-| `openspec` | Use OpenSpec as the source of truth for planning, implementation, verification, and archive discipline. | user | `/home/alex/.config/opencode/skills/openspec/SKILL.md` |
 | `openspec-apply-change` | Implement tasks from an OpenSpec change. Use when the user wants to start implementing, continue implementation, or work through tasks. | project | `/home/alex/projects/headquarter/.opencode/skills/openspec-apply-change/SKILL.md` |
 | `openspec-archive-change` | Archive a completed change in the experimental workflow. Use when the user wants to finalize and archive a change after implementation is complete. | project | `/home/alex/projects/headquarter/.opencode/skills/openspec-archive-change/SKILL.md` |
 | `openspec-explore` | Enter explore mode - a thinking partner for exploring ideas, investigating problems, and clarifying requirements. Use when the user wants to think through something before or during a change. | project | `/home/alex/projects/headquarter/.opencode/skills/openspec-explore/SKILL.md` |