feat(webhooks): docs + benchmark + finish package wiring (#190)

[tonydspaniard]

Part of #184 (webhook epic). Closes #190. Follows #189 (merged in #192).

The issue was scoped as "docs + benchmark," but writing the docs surfaced two real wiring gaps the merged sub-issues (#185–#188) left behind. Per the agreed approach, this PR fixes them so the documented surface is real.

Wiring gaps fixed

• No WebhooksConfiguration — added one mirroring IdempotencyConfiguration: binds the in-memory dedupe + delivery-store adapters, EnvSecretResolver, and a shared SignerRegistry::default(). Hosts swap to Redis by re-binding the interfaces.
• CLI commands were dead code — WebhookReplayCommand / WebhookShowFailedCommand (WebhookDispatcher + Messenger handler: outbound POST with retry / dead-letter / replay #187) were written in Symfony Console style (#[AsCommand], extending Symfony Command), which the Altair AttributeCommandDiscoverer never finds. Ported both to the Altair #[Command]/#[Argument]/#[Option] convention and registered src/Altair/Webhooks/Cli in bin/altair. They now appear in bin/altair list (autowire-fail under bare bin/altair exactly like db:*, resolving once the host applies WebhooksConfiguration).

Docs

• docs/packages/webhooks.md — full reference mirroring idempotency.md: intro, install, inbound + outbound quick starts, signing primitives, storage adapters, both behaviour matrices, auto-wiring, round-trip, limits.
• src/Altair/Webhooks/README.md — slim README pointing at the doc.
• docs/README.md — indexed Webhooks (and the previously-missing Idempotency) under the HTTP stack section.

Benchmark

• benchmarks/tokens-to-ship/task-webhooks.md — inbound + outbound variant, reported as two separate rows.
• fixtures/posts-webhook-in.openapi.yaml + posts-webhook-out.openapi.yaml — both round-trip clean through openapi:roundtrip and scaffold a working project through openapi:import --scaffold (verified: recovered spec carries the webhook: block; generated Action exposes webhook()).

Test plan

• WebhooksConfigurationTest + rewritten CLI command tests (direct __invoke + output capture)
• PHPStan L8 / CS / Rector clean cache-free
• Full webhooks + scaffold suites green (ext-redis tests skipped locally, run on CI)