Closes #146

Summary

• Adds a top-level slow-run summary to Diagnostics that names the most likely driver: tool latency, long gap, retry loop, large parameters, context pressure, or no dominant driver.
• Adds an ordered Slow Path evidence panel before lower-priority diagnostic cards.
• Sorts per-tool latency by P95/max latency so slow tools appear first.

User value

Users can open Diagnostics and immediately see the likely explanation for a slow run, with the supporting evidence ordered before secondary sections.

Scope

• TUI diagnostics layout and copy only.
• English/Chinese labels for slow-run summary and slow-path evidence.
• Regression coverage for slow-run explanation ordering.

Changed files

• internal/tui/tui.go
• internal/tui/tui_test.go
• internal/i18n/i18n.go

Test plan

• go test ./internal/tui
• go test ./...
• go build -o /tmp/agenttrace-parser-check ./cmd/agenttrace
• /tmp/agenttrace-parser-check --doctor || true
• /tmp/agenttrace-parser-check --demo --overview -f json
• go test ./internal/tui -run TestDiagnosticsRendersSlowRunExplanation -count=1 -v

Fixture/privacy note

No real user session data, prompts, logs, tokens, or secrets were added. Tests use existing synthetic in-memory TUI fixtures. Local doctor validation saw 4267 session files, but no local session content was committed.

Risk

Low-medium. This changes Diagnostics visible ordering and narrative copy. Parser/source compatibility, CLI behavior, and JSON/Markdown/HTML report contracts are unchanged.

Non-goals

• No parser/source changes.
• No report contract changes.
• No hosted tracing or eval framework scope.
• No README/docs asset changes.

Blackboard events

• Claimed [TUI] Strengthen diagnostics as a slow-run analyzer #146 with: Parser & Implementation Agent is taking this.
• Created and pushed branch parser/146-diagnostics-slow-run.
• Implemented slow-run summary, ordered slow-path evidence, and latency ranking.
• Terminal capture evidence: focused diagnostics test passed with -v.

Release impact

patch; affects TUI visible behavior only. Parser/source compatibility: no. CLI behavior: no. JSON/Markdown/HTML report contract: no. Install/package docs: no.

Handoff suggestion: Growth

Suggested release note: Diagnostics now leads with a slow-run explanation and ordered evidence for latency, gaps, loops, parameters, and context pressure.