Show a run summary with steps, time, tokens, and cost #15

Problem

tinyagent tracks step count and cost while it runs, and -o saves them, but the CLI never reports them. A successful run ends with nothing but the model's streamed text; a failed one prints <Status> after N steps and a hint. Users cannot tell how long a run took, how many tokens it used, or what it cost without opening the trajectory JSON — and that JSON does not even include token counts today, because the loop only accumulates Cost.USD.

This matters most for the limits that already exist: --step-limit, --cost-limit, and --time-limit. There is no feedback on how close a run came to them, and an expensive run gives no cost readout at the end.

Proposed behavior

Print one compact summary line to stderr whenever a run ends, for every exit status. Keep stdout for the model's text only, so redirecting it still yields a clean answer.

Examples:

Submitted after 12 steps in 1m42s (48,213 input / 3,104 output tokens, $0.23)

StepLimitExceeded after 50 steps in 6m03s (120,441 input / 11,209 output tokens, $1.87)

Error after 3 steps in 12s (4,120 input / 280 output tokens, $0.0043): HTTP 401: Incorrect API key

• Status, steps, duration, and token counts are shown whenever they are known.
• Cost appears only when it is greater than zero, so Codex subscription runs and unknown-priced models do not print a misleading $0.00. Format costs at or above one cent with 2 decimals, below one cent with 4.
• Duration is wall-clock and human-readable (12s, 1m42s, 1h02m).
• If a run ends before any model response (for example Ctrl-C during the first call), omit the token/cost parenthetical rather than printing zeros: Interrupted after 0 steps in 2s.
• Keep the existing hint: line for setup errors.

Scope

• internal/trajectory/trajectory.go: add InputTokens and OutputTokens fields (json:"input_tokens", json:"output_tokens"). This is additive, so existing consumers keep working.
• internal/agent/loop.go: accumulate cost.InputTokens and cost.OutputTokens alongside t.Cost.
• cmd/tinyagent/main.go: track the run's start time and print the summary for both success and failure. Keep exit codes and stdout behavior unchanged.
• cmd/tinyagent/main_test.go: table-driven tests for the formatter (submitted, limit, error, zero cost, zero steps).
• internal/agent/loop_test.go: assert token totals land in the trajectory.
• README.md: mention the summary under "Exit status".

Acceptance criteria

• Every run — Submitted, Completed, StepLimitExceeded, CostLimitExceeded, TimeLimitExceeded, Interrupted, Error — prints exactly one summary line to stderr.
• The summary includes status, steps, elapsed time, and input/output token totals.
• Cost is printed when non-zero and omitted at zero.
• stdout still contains only the model's text; exit codes are unchanged.
• Trajectories written with -o include matching input_tokens/output_tokens totals; existing JSON fields are unchanged.
• go test ./... passes, and the README documents the new line.

This is a small, additive change: providers already return token counts in model.Cost, so it only extends end-of-run reporting and adds two trajectory fields.