Review docs/api.md and docs/llm_posting_helper.md for accuracy and completeness

Same pass as the SKILL.md review. Fixed factually-wrong content and filled
gaps against the current code.

docs/api.md:
- Capabilities example response was stale: artifact_kinds missing json/csv,
  raw_supported_for missing json/csv. Corrected to match live output, with
  a note that the endpoint is the authoritative source.
- "Office document conversion is not implemented" is now false - replaced
  with the accurate scope (.xlsx -> csv only; .docx/.pptx/.xls not).
- Source Fields overview table omitted source_prompt entirely; added it,
  plus a note that source_name/source_id/source_prompt must be strings
  (400, not coerced) and metadata must be an object.
- Kinds table: noted tables now render styled in markdown/html_fragment,
  and made explicit that html_full is served verbatim with no styling.
- MVP constraints: noted the public-share-link exception to "feeds private
  per user" and clarified PATCH edits metadata only.

docs/llm_posting_helper.md:
- Added .xlsx upload, --upload-image image embedding, a Source & Theme
  Flags table (all six source-*/theme flags were undocumented here), and a
  note that --meta/--meta-json merge and are distinct from tags.
- Added a scope note that the helper only creates - update/delete need the
  API, sharing is feed-UI-only.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/llm_posting_helper.md

@@ -6,6 +6,11 @@ The project also keeps `scripts/daybook_post.py` as a compatibility wrapper.
 
 The helper is intentionally zero-dependency and uses only Python's standard library. It works on Windows and Linux.
 
+It only **creates** artifacts. Updating (title/tags/metadata) and deleting
+require the raw API (`PATCH`/`DELETE /api/artifacts/{id}`, see `docs/api.md`);
+enabling a public share link is done from the feed UI and has no API or CLI
+path yet.
+
 ## Configuration
 
 Set these as real process/shell environment variables — the helper reads
@@ -64,6 +69,13 @@ python skills/daybook-posting/scripts/daybook_post.py results.csv
 python skills/daybook-posting/scripts/daybook_post.py --kind json --title "Run Summary" --content-file summary.json
 ```
 
+Post an `.xlsx` spreadsheet (converted server-side to a `csv` artifact —
+active sheet only, saved formula results only, no formatting preserved):
+
+```bash
+python skills/daybook-posting/scripts/daybook_post.py balance_sheet.xlsx
+```
+
 Render other code as a Markdown fenced block instead (no dedicated `code`
 kind yet):
 
@@ -71,6 +83,14 @@ kind yet):
 python skills/daybook-posting/scripts/daybook_post.py --file script.py --as-markdown --title "Generated Script"
 ```
 
+Embed an image inside a markdown/HTML artifact (upload first, then
+reference the printed URL in your content):
+
+```bash
+python skills/daybook-posting/scripts/daybook_post.py --upload-image chart.png
+# -> prints .../assets/<id>; put that in your markdown, then post normally
+```
+
 Dry run:
 
 ```bash
@@ -112,6 +132,21 @@ Or pass a JSON object:
 python skills/daybook-posting/scripts/daybook_post.py --kind markdown --title "Run Summary" --content-file summary.md --meta-json '{"run_id":"run-001"}'
 ```
 
+`--meta` and `--meta-json` are merged if both are given. Metadata lives in
+the artifact's `metadata` field, separate from `--tags` (which are for
+filtering).
+
+## Source & Theme Flags
+
+| Flag | Default | Notes |
+|---|---|---|
+| `--source` | `DAYBOOK_SOURCE` env, else `agent` | What produced this — an agent-profile, script, or person name. |
+| `--source-type` | `agent` | One of `agent`, `person`, `system`. |
+| `--source-name` | The effective `--source` | Human-readable name, e.g. "Hermes Agent". |
+| `--source-id` | Empty | A stable external ID, if one exists. |
+| `--source-prompt` | Empty | The prompt/instruction behind the artifact. |
+| `--theme` | Kind default | `markdown`/`html_fragment` only: `report` (default), `tasks`, or `base`. Other kinds ignore it. |
+
 ## Skill Wrapper
 
 A thin LLM-facing wrapper lives at: