[Feature] Capability plugin SPI foundation, with server-time as the first module #100

Summary

First of four PRs for #98 (design: specs/014-capability-plugin-spi/). Adds the capability plugin SPI to core and moves server-time onto it as the first module, so the SPI is proven end to end on the simplest capability.

Scope

• CapabilityExtension.register(CapabilityRegistrar) and the typed registrations the later migrations need (events, inbound/outbound tags, recipients, member status and NAMES identity, reply fields, output groups, commands), with CapabilityDescriptor dependencies.
• Core dispatch for those hooks, the defined failure policy for a throwing hook, and the reply-budget accounting for hooks that change reply sizes.
• Move server-time from contributeTags to the new outbound-tag registration. contributeTags and includeSenderInFanOut stay only until [Feature] Move message-tags, echo-message, away-notify, multi-prefix and batch onto the capability SPI #101 removes their last callers.
• The spec docs for feature 014 land with this PR.

Acceptance

• server-time behaviour lives only in its module; wire output is unchanged.
• Existing integration tests pass unchanged.
• Unit tests for each registration type and for hook failure isolation.

Part of #98.