Support multiple external agent import sources

[charlesgong-openai]

Summary

• route external configuration detection and import through source-specific adapters while preserving the existing default
• complete the secondary adapter's config, cached marketplace plugin, compatible hook, and recent session migrations
• rewrite only the exact title-cased source name while preserving lowercase and acronym uses
• recognize the secondary marketplace and plugin manifest layouts, including safe bare relative component paths

Why

The migration path was tied to one source layout and the secondary adapter covered only a subset of import categories. This change makes the adapter boundary complete enough to migrate the supported customization surfaces without changing default requests.

User impact

Users can detect and import supported home- and repository-scoped configuration from either adapter. Existing target values remain authoritative, unsupported hook constructs are skipped, session discovery stays bounded, and the original source files are not modified.

Validation

• focused migration, session, marketplace, and plugin utility suites: 382 tests passed
• app-server external configuration suite: 62 tests passed
• app-server protocol suite: 256 tests passed
• dependency lock refresh completed
• scoped Clippy fixes, formatting, and final diff checks passed