feat(workflows): drop the Tool Execution Flow sankey

The d3-sankey tool-transition chart was the last of the deep harness-internals
visualizations issue #6 set out to remove: a wall of bands showing that Read
tends to precede Edit, with nothing a reader could act on. It goes, along with
its i18n keys in both locales and the now-orphaned d3-sankey dependency.

The workflows API response is untouched — `toolFlow` still ships in the
aggregate and still feeds the Dashboard's top-tools list. Remaining sections
renumber 4-9.

client/src/i18n/locales/en/workflows.json

@@ -58,11 +58,6 @@
       "howToRead": "Each column is one layer. Box width is constant; the small badge is the count. Edges flow left-to-right; thicker edges = more frequent transitions. Hover any node for stats; click to filter the rest of the page by it.",
       "why": "Use it to answer 'where does work go in my workflows?' - which subagent types dominate, where compaction kicks in, and which paths end in errors vs clean completions."
     },
-    "toolFlow": {
-      "what": "A Sankey-style chart of consecutive tool-call transitions across every session in scope. Each band is one (source → target) pair counted across all sessions.",
-      "howToRead": "Tools appear as colored bars on the left and right. Bands connect them: thickness = number of times the source ran immediately before the target. Hover a band for share-of-source and share-of-target percentages; hover a node for its total usage and share of all tool calls.",
-      "why": "Surfaces habitual tool chains (e.g. Read → Edit → Bash). Useful for spotting loops, wasted hops, and missing tool variety in agent behavior."
-    },
     "drillIn": {
       "what": "Per-session detail view: full agent tree, tool-call timeline, and raw event sequence for whichever session you select.",
       "howToRead": "Pick a session from the dropdown above or click a bubble in the scatter plot. The tabs let you switch between hierarchical (Agent Tree), temporal (Tool Timeline), and chronological (Event Sequence) views of the same session.",
@@ -221,24 +216,6 @@
       }
     }
   },
-  "toolFlow": {
-    "title": "Tool Execution Flow",
-    "subtitle": "How tools chain together - Sankey-style directed flow",
-    "noData": "No tool flow data",
-    "transitionCount_one": "{{count}} transition",
-    "transitionCount_other": "{{count}} transitions",
-    "tooltip": {
-      "node": "Tool node",
-      "link": "Tool transition",
-      "totalCalls": "Total calls",
-      "shareOfAll": "Share of all tool use",
-      "transitionsObserved": "Transitions observed",
-      "shareOfSourceFmt": "Share of {{source}} → next",
-      "shareOfTargetFmt": "Share of prev → {{target}}",
-      "nodeDescFmt": "Bar height shows how often {{name}} appears across all sessions in scope. Bands flowing in on the left show what ran before {{name}}; bands flowing out on the right show what ran after.",
-      "linkDescFmt": "Across every session, this counts how often {{source}} ran immediately before {{target}}. Thicker bands = more common transitions; the percentages show how dominant this hop is among each tool's outgoing / incoming flow."
-    }
-  },
   "drillIn": {
     "title": "Session Drill-In",
     "subtitle": "Select a session from the scatter plot or use the dropdown to explore",