fix(task-board): pluralize the task-cost run count correctly

[pedrofrxncx]

#6154 (merged today) shipped the task-cost chip with a single hardcoded string for the run count and tooltip: "in {runs} runs" / "em {runs} execuções". A task with exactly one linked run — the common case, a card that's just its Super Agent run with no reviewer/re-run rounds — reads "in 1 runs", and the pt-br tooltip has the identical singular/plural mismatch ("1 execuções").

A maintainer wants this fixed because it's user-visible copy shipped hours ago that reads wrong for the majority of cards (single-run tasks are the default, not the exception).

Failure scenario: open any task-board card whose only linked run recorded a cost. The chip currently shows $X.XX in 1 runs. After the fix it shows $X.XX in 1 run.

Fix: split taskBoard.taskDialog.costRunCount / costTooltip into ...Singular / ...Plural key pairs (en + pt-br) and pick by runs === 1 in TaskCost — the same pattern already used by settings.apiKeys.keysCount(Singular|Plural) and settings.secrets.secretsCount(Singular|Plural) elsewhere in this codebase.

To confirm: open a task-board card whose only run has recorded cost — the chip reads "in 1 run", not "in 1 runs".

Checked locally: bun run fmt (clean), cd apps/web && bunx tsc --noEmit (clean — pt-br's satisfies Record<keyof typeof enDomain, string> catches any key mismatch at compile time), bunx oxlint on the three touched files (0 warnings/errors). No unit test exists for this component; the change is UI copy only. Full CI validates the rest.

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Summary by cubic

Fixes the task-board cost chip to pluralize the run count and tooltip correctly. Previously it always showed “in {runs} runs” (e.g., “in 1 runs”); now it selects singular/plural for both the label and tooltip. No behavior change beyond copy.

• Split i18n keys into taskBoard.taskDialog.costRunCountSingular/Plural and taskBoard.taskDialog.costTooltipSingular/Plural in en and pt-br; removed the old costRunCount/costTooltip keys.
• TaskCost picks singular when runCount === 1; run counting and cost calculation logic are unchanged.
• Verify by opening a task with a single run and recorded cost: chip reads “in 1 run” (pt-br: “em 1 execução”). No migrations required.

^{Written for commit 06abbcf. Summary will update on new commits.}