fix(subagent): strip MessagePart::Image from transcript writes

[bug-ops]

Summary

• TranscriptWriter::append serialized the full Message verbatim to the sub-agent's <task_id>.jsonl transcript with no filtering. Sub-agents never call Agent::persist_message, so their transcripts bypassed the MessagePart::Image strip landed in fix(core): strip ephemeral Image parts before persistence writes #6307 for spec-072 §4 C1 entirely — a defense-in-depth gap, not currently exploitable, but reachable once Implement: P2 MediaSanitizer decode/validate/attach + vision-tier routing for MCP image passthrough #6240 (MCP image emission in tool-result processing) lands.
• Extracted the strip logic into a shared MessagePart::strip_images helper on zeph-llm's MessagePart (avoids duplicating the filter and avoids a circular dependency, since zeph-common cannot host it while zeph-llm already depends on zeph-common). Both Agent::persist_message and TranscriptWriter::append now call this shared helper.
• The caller's in-memory Message is unaffected; TranscriptWriter::append strips a clone before serializing.

Closes #6305

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml -p zeph-subagent -p zeph-llm -p zeph-core --lib --bins (3381 passed)
• cargo test --doc -p zeph-llm (25 passed, including the extended strip_images doctest)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged --no-banner --redact — no leaks
• New tests: image parts stripped from written/read-back transcript, non-image parts pass through unchanged, empty-parts messages unaffected, mixed Text+Image case
• Adversarial critique (impl-critic) and independent code review both completed — approved