feat(ui): unify Chat + Session into one Chat tab with a session-mode toggle (#1112)

[dolho]

Summary

Collapses the redundant Chat + Session tabs on Agent Detail into a single "Chat" tab carrying a "Session mode" toggle (default ON). The legacy stateless surface stays a first-class, user-selectable mode rather than dead code. theme-ui-ux. No backend change.

Behavior

• Toggle ON → SessionPanel.vue (--resume continuity: memory, tool-result + reasoning state across turns). OFF → legacy stateless ChatPanel.vue. Swaps in-place (v-if on effectiveChatMode), no page nav.
• Persistence: per-user via localStorage['trinity.chatMode'], default session.
• Feature-flag / runtime fallback: when session_tab_enabled is off or the runtime lacks --resume (Codex, Codex harness MVP — pluggable agentic execution engine alongside Claude Code #1187), the toggle is hidden and the tab renders legacy ChatPanel only — never zero chat surfaces.
• Fullscreen: isFullscreenTab now keys on the single 'chat' id (both modes).

Routing

• ?tab=session aliases to the chat tab (TAB_ALIASES) and hints session mode; ?tab=chat resolves as before. Both land on the unified tab.
• ExecutionDetail "continue as chat" (?tab=chat&resumeSessionId=…) forces legacy ChatPanel (which owns resumeSessionId) for that landing via a transient, non-persisted routeForcedMode — so an execution-resume doesn't rewrite the user's saved preference. The toggle reflects the forced mode and clicking it clears the override.

Acceptance criteria

• Exactly one tab labeled "Chat" (never two, never zero)
• "Session mode" toggle, default ON, with explanatory tooltip
• ON → SessionPanel, OFF → ChatPanel, in-place swap
• Choice persists (per-user localStorage, documented)
• Feature-flag fallback → legacy only, toggle hidden
• ExecutionDetail deep-link lands on the unified tab; execution-resume forces legacy (defined, no dead tab)
• ?tab=chat and ?tab=session both resolve to the unified Chat tab
• No regression to other tabs or isFullscreenTab (single id, both modes)

Notes

• MobileAdmin needed no change: its openChat is a self-contained mobile chat overlay (API-driven, /chat/sessions + /task), not an AgentDetail tab deep-link — so the tab unification can't land it on a dead surface. (The issue listed it as a deep-link; the code shows otherwise.)
• Scope decision: mode preference is per-user (one localStorage key) rather than per-agent — simpler, and the toggle reads as a workflow preference.

Validation

SFC compiles (vue/compiler-sfc parse + compileScript); design-token check passes. Full vite build is blocked locally only by an unrelated missing mermaid dep in AgentWorkspace.vue (declared in package.json, absent from this checkout's node_modules) — CI builds with full deps. UI e2e (ui label) exercises the live surface.

Related to #1112

🤖 Generated with Claude Code

[vybe]

Validated via /validate-pr: base=dev, conventional commit, focused UI feature (AgentDetail.vue) with architecture.md + requirements.md both updated. Security-clean, no enterprise-doc leak in arch hunk, build green. Note: e2e not triggered (no ui label) — relying on build + thorough acceptance criteria. Approving.