feat(event): add canonical Usage event builder, adapter contract, and dep bump

[chgeuer]

Closes #4

What

Adds Jido.Harness.Event.Usage.build/3 — a builder that constructs canonical :usage events with a standardized flat payload. Also extends AdapterContract to verify that adapters declaring usage?: true emit at least one conforming :usage event.

Canonical payload shape

%{
  "input_tokens"        => non_neg_integer(),   # required
  "output_tokens"       => non_neg_integer(),   # required
  "total_tokens"        => non_neg_integer(),   # computed if absent
  "cached_input_tokens" => non_neg_integer(),   # optional, default 0
  "cost_usd"            => float() | nil,       # optional
  "duration_ms"         => integer() | nil,     # optional
  "model"               => String.t() | nil     # optional
}

Key design decisions:

• Flat payload (no nested "usage" key) — simpler for consumers to pattern-match
• One event per session, emitted just before :session_completed
• session_id required — enables session-level aggregation

Usage

alias Jido.Harness.Event.Usage, as: UsageEvent

UsageEvent.build(:claude, "session-42",
  input_tokens: 1250,
  output_tokens: 890,
  cost_usd: 0.00542,
  model: "claude-sonnet-4-20250514"
)

Changes

• lib/jido_harness/event/usage.ex — new module with build/3 and @moduledoc documenting the contract
• lib/jido_harness/adapter_contract.ex — new contract test: adapters with usage?: true must emit :usage events with input_tokens, output_tokens, total_tokens, and a non-nil session_id
• test/jido_harness/event/usage_test.exs — 8 unit tests covering required/optional fields, defaults, and validation
• mix.exs — bumps jido dep to ~> 2.1

Test results

67 tests, 0 failures