Retire the ECHOLOT and Subject Sources planning docs

[JeroenDeDauw]

Both docs restated ADRs 22, 23, 28 and 35, the reference docs and open issues, and parts of them had gone stale. Their open questions are tracked elsewhere now.

ECHOLOT.md:

• Statement-level requirements: qualifiers-and-references.md
• Multilinguality and the WP4 boundary: ECHOLOT requirements: NeoWiki implementation status #1293
• Schema subclasses: ADR 28's out-of-scope list
• Provenance boundary: Q5 in NativeRdfProjection.md
• Multi-Subject editing: ADR 35 and Implement Dependent Subjects #1554
• Rights statement selector and concurrent writes, which had no issue: ECHOLOT requirements: NeoWiki implementation status #1293
• Bulk ID minting has shipped; one Schema per Subject is ADR 8

SubjectSources.md:

• Model, identity, Page Properties and graceful degradation: ADR 23 and ADR 22
• Materialising sourced Subjects, Source contract gaps, farm source keys and frontend source-awareness: Sources have no way to materialise their Subjects into the graph #1370, Source contract gaps an adapter author hits (Schema construction, sourced-Schema fetch, id escaping, Schema IRIs) #1371, Registering a farm sibling whose Wiki ID is not a valid Source key fails fatally #1372 and Frontend source-awareness: display sourced Subjects and Schema references, enter cross-source relation targets #1373
• Federation resolution and write-back: ADR 23's open questions
• History rendering of sourced Subjects, which had no issue: Render historically accurate on history pages #952

ADR 22 and ADR 23 keep their pointer to the exploration, now a permalink to its last version.

_{AI-authored — Claude Code, Opus 5.5 (1M context) (max); asked for by @JeroenDeDauw after an in-session audit of the planning docs, no revisions; diff not yet human-reviewed; inbound links checked with git grep.}