feat: v0.4.0 — rich content support with typed blocks and loss visibility

Extracts per-message content into a typed `blocks` list (text, code,
thinking, tool_use, tool_result, image_placeholder, file_placeholder,
unknown) and renders them at exporter write time. Voice transcripts,
Custom Instructions, and image references now appear in exports
instead of being silently dropped.

Foundation:
- src/blocks.py: pure block constructors, _safe_fence (fence-corruption
  defense, verified live in Joplin), _blockquote_prefix, render
- src/loss_report.py: per-run tally surfaced as INFO summary at end of
  export so silently-dropped data becomes visible

Providers:
- ChatGPT: dispatch on content_type produces typed blocks; voice shapes
  (audio_transcription, audio_asset_pointer, real_time_user_audio_video_
  asset_pointer) locked from live DevTools capture; Custom Instructions
  bug fix (parts-vs-direct-fields); role filter lifted; hidden-context
  marker driven by is_visually_hidden_from_conversation flag
- Claude: defensive dispatch for text/thinking/tool_use/tool_result/image
  with recursive nested-block flattening; untested against real rich-
  content data — fix-forward in v0.4.1

Exporter:
- Markdown renders from blocks at write time via render_blocks_to_markdown;
  backward-compat fallback to content for any pre-v0.4.0 cached data

Tests:
- 27 new tests across providers, exporters, CLI; fixtures rebuilt with
  real-shape ChatGPT voice + Custom Instructions cases
- 181/181 pass

Behavior changes (intentional):
- JSON output omits content; consumers should read blocks
- Per-conversation message counts increase (Custom Instructions, image-
  only, tool-only messages now appear)
- Existing exports not auto-re-rendered; users wanting fresh output run
  cache --clear then export

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -426,7 +426,7 @@ Make sure you've added the project IDs to `CHATGPT_PROJECT_IDS` in your `.env`.
 The provider's internal API may have changed. Run with `--debug`, sanitize the output (remove any personal content), and check the project's GitHub Issues for known fixes.
 
 ### Non-text content warnings
-Images, code interpreter outputs, DALL-E generations, and Claude artifacts are not exported in v0.2.0. A WARNING is logged for each skipped item. See `FUTURE.md` for the roadmap.
+Since v0.4.0, rich content is preserved as typed blocks in the export. ChatGPT voice transcripts render as text and audio assets as `📎 File attached` placeholders with size and duration metadata. Images render as `🖼️ Image attached` placeholders showing the asset reference. Custom Instructions appear under a `> ℹ️ Hidden context` marker. Anything the extractor doesn't recognise renders as a visible `> ⚠️ Unsupported content` block naming the type and observed keys, *and* increments a counter in the post-export summary so you can tell whether real content is being silently skipped. Binary downloads (the actual image/audio bytes) are still deferred — see `FUTURE.md` v0.5.0.
 
 ### Empty export / all conversations skipped
 No new or updated conversations since your last run. To verify: `ai-chat-exporter cache --show`. To force a full re-export: `ai-chat-exporter cache --clear`.
@@ -444,7 +444,7 @@ See `FUTURE.md` for planned features:
 
 - **v0.2.x** — `export --force` flag; `joplin --force` flag; per-conversation cache reset
 - **v0.3.0** — Official API fallback: parse export ZIP files from ChatGPT/Claude settings
-- **v0.4.0** — Rich content: images, artifacts, code interpreter output, extended thinking
+- **v0.4.x / v0.5.0** — Binary content downloads (images, audio bytes) and Joplin resource upload; reclassify o1/o3 reasoning subparts; optional `EXPORTER_INCLUDE_HIDDEN_CONTEXT` toggle
 - **v0.5.0** — Watch/scheduled mode; Obsidian vault output
 
 ---