[vicgupta]

Closes #15

Summary

• Every run now ends with one summary line on stderr (stdout stays model text only; exit codes unchanged), e.g. Submitted after 12 steps in 1m42s (48,213 input / 3,104 output tokens, $0.23).
• Cost is shown only when > 0 (2 decimals from one cent up, 4 below). The token/cost parenthetical is omitted when no model response arrived (Interrupted after 0 steps in 2s). Errors append : <message> and keep the existing hint: line.
• Durations render as 12s, 1m42s, 1h02m.
• trajectory.Trajectory gains additive input_tokens / output_tokens fields; the agent loop accumulates them alongside cost.
• reportFailure is replaced by reportSummary, called for every exit status.
• README documents the summary line under Exit status.

Tests

• New table-driven TestSummaryLine (submitted, limit, error, zero cost, zero steps) and TestReportSummary (error + hint, success).
• TestRunSubmits asserts token totals land in the trajectory.
• go build ./..., go vet ./..., go test ./... all pass.