fix(monitoring): scale tool-call heatmap colors by displayed cells only

[pedrofrxncx]

Follows #5865 (tool-call heatmap by agent), which merged just before this tick.

Bug: ToolAgentHeatmap picks the top 12 tools and top 8 agents by total volume, then colors each cell with heatmapLevelClass(value, maxValue). But maxValue was computed by scanning every cell that matched the top-12 tools, without also restricting to the top-8 agents actually rendered. An agent that gets filtered out of the top 8 (low total across all its tools) can still have a single outlier cell higher than anything shown in the rendered grid — that outlier silently becomes the color-scale ceiling, so every displayed cell renders duller than its real share of the visible data, including the actual max cell shown.

Fix: compute maxValue only over cells whose tool AND agent both made the top lists — i.e. the same set that's actually rendered in the grid.

How to verify: cd apps/web && bunx tsc --noEmit (no errors touching overview.tsx) and bunx oxlint apps/web/src/routes/orgs/monitoring/overview.tsx (0 warnings/errors). This is a pure rendering-logic fix with no new dependency or schema change; full CI validates the rest.

Checked locally: bun run fmt, targeted tsc --noEmit in apps/web, bunx oxlint on the changed file.

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Summary by cubic

Fix heatmap color scaling in ToolAgentHeatmap by normalizing against only the displayed cells (top 12 tools × top 8 agents). This prevents hidden outliers from setting the scale and makes visible cells reflect their true relative values.

• Bug Fixes
  • Compute the color-scale max using only cells whose tool and agent are in the rendered top lists; previously included filtered agents, which washed out colors.

^{Written for commit 3778148. Summary will update on new commits.}