docs(specs): enumerate sub-agent transcripts as 4th persistence surface in spec-072

[bug-ops]

Summary

• spec-072 §4 (invariant C1, FR-008, AC-5, threat model) previously enumerated only three persistence surfaces requiring MessagePart::Image stripping: SQLite parts_json, Qdrant embeddings, and the durable JSONL session log.
• PR fix(subagent): strip MessagePart::Image from transcript writes #6310 added a fourth strip site — sub-agent transcripts (TranscriptWriter::append in crates/zeph-subagent/src/transcript.rs, calling MessagePart::strip_images) — but the spec text was never updated to reflect it.
• This is a doc-only change: updates all five affected sections of spec.md (C1 enumeration, C1 scope, FR-008, AC-5, threat model) plus a CHANGELOG.md entry. No source files touched.

Closes #6311

Test plan

• Verified TranscriptWriter::append calls MessagePart::strip_images before writing (crates/zeph-subagent/src/transcript.rs:127-129, crates/zeph-llm/src/provider.rs:369)
• Verified zeph-subagent does not route through Agent::persist_message, confirming the "structurally separate path" claim
• Grepped the full spec file for stale "three persistence surfaces" language — none remains
• git diff --stat confirms only specs/072-multimodal-mcp-passthrough/spec.md and CHANGELOG.md changed
• Line-wrap convention verified for the edited C1 paragraph (all lines within surrounding ~90-102 char convention)