refactor(flow): let each step kind own how it reads

[latekvo]

Stacked on #574.

Summary

summarizeSteps (flow-finish-recording.ts) was a 50-line switch over FlowStep["kind"]. stepTarget (flow-run.ts) was a second one, over the same union, with its own selector and condition spellings. Neither could be found from the other, and summarizeSteps' case "tool": default: arm meant a step kind added without wording rendered as a tool: step instead of failing.

Both surfaces now read from one definition per kind — the same shape as a tool owning its own log wording in ToolDefinition.interaction (#582):

const FLOW_STEP_DEFINITIONS: {
  [K in FlowStep["kind"]]: FlowStepDefinition<Extract<FlowStep, { kind: K }>>;
} = {
  "scroll-to": {
    summary: (step) => `${yamlSelectorLabel(step.target)} (${step.direction})`,
    target: (step) => { ... },
  },
  ...
};

Keying the record by FlowStep["kind"] is what replaces the default arm: a new kind is a compile error until it says how it reads on both surfaces. That also retires the renderToolArgs interpolation guard, which existed only because the default arm could hand it a step with no args field.

flow-run.ts sheds 98 lines and six imports; flow-finish-recording.ts sheds 118 and is now just the tool. The two shared label templates (conditionLabel parameterized by selector spelling, and a new whenLabel for the platform-vs-UI guard arm) mean the recorder's when: tail and the runner's no longer duplicate the platform arm.

Test

Rendering is unchanged — every summary line and report target is byte-for-byte what the switches produced.

• Proved with a throwaway equivalence test that inlined both pre-refactor switches verbatim and compared them against the new definitions over 1264 generated steps: all 15 kinds, 10 selector shapes (loose, regex, scoped, quote-bearing), every wait condition × text-match mode × expectation, every scroll direction, and the cyclic-args path. Identical on all of them. Mutating two renderings (equals comparator, default scroll direction) failed it, so it discriminates.
• Replaced by packages/tool-server/test/flows/flow-step-definitions.test.ts, kept: 47 assertions pinning the exact summary line and report target of every kind, in a table keyed by FlowStep["kind"] so a new kind fails typecheck:tests here too (verified by deleting the wait entry: TS1360). Deleting a kind from the definitions record fails the build the same way (TS2741).
• npm run build, eslint --max-warnings 0, prettier --check ., npm run typecheck:tests --workspaces: clean.
• packages/tool-server: 311 files / 3596 tests passed. packages/argent-cli: 24 files / 461 tests passed.

Note this PR targets feat/concurrent-flow-recordings, and the Unit Tests workflow only runs on PRs into main — the runs above are the signal until #574 lands.