fix: reduce border weight on tool call cards

[broodcicada-glitch]

What

Reduces border weight on tool call cards in the chat UI.

Before/After

Tool execution cards had heavy borders with shadows. After: lighter borders, less visual noise.

Files: 1 changed (ToolCallBlock.tsx)

[github-actions]

PR quality triage

Changed areas: area:desktop

CLI core policy: No CLI-core policy block detected.

Missing-test policy: Blocked by policy until a maintainer applies allow-missing-tests or matching tests are added.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• none

Coverage policy files:

• none

Required check plan:

• See PR Quality / scope-plan; it is the single source of truth for selected jobs.

Test coverage signals:

• BLOCKING unless allow-missing-tests is applied: Desktop product files changed without a desktop test file in the PR.

Risk notes:

• No special risk notes from changed paths.

Hard merge gates come from the deterministic GitHub Actions contract lanes above.