Implement artifact-level render command with Markdown output and update docs

docs/cli.md

@@ -16,6 +16,7 @@ go run ./cmd/seriatim merge \
 | `merge` | Merge one or more raw transcript JSON inputs into one seriatim artifact. |
 | `trim` | Keep or remove segment IDs from an existing seriatim artifact. |
 | `normalize` | Canonicalize transcript-like JSON into a seriatim artifact. |
+| `render` | Render an existing seriatim artifact as Markdown. |
 
 Root usage:
 
@@ -130,6 +131,34 @@ Flags:
 - Does not run merge modules.
 - When `--output-schema` is omitted, schema resolution is: `SERIATIM_OUTPUT_SCHEMA` -> default `seriatim-intermediate`.
 
+## `render`
+
+Usage:
+
+```text
+seriatim render [flags]
+```
+
+Flags:
+
+| Flag | Required | Default | Description |
+| --- | --- | --- | --- |
+| `--input-file string` | Yes | none | Input seriatim artifact JSON file. |
+| `--output-file string` | Yes | none | Rendered output file path. |
+| `--format string` | Yes | none | Output format. Current supported value: `markdown`. |
+| `--title string` | No | `Transcript` | Markdown document title. |
+| `--include-timestamps` | No | `true` | Include `[HH:MM:SS–HH:MM:SS]` per segment. |
+| `--include-segment-ids` | No | `false` | Include `[#id]` marker per segment. |
+| `--include-metadata` | No | `false` | Include artifact metadata block near the top. |
+
+`render` behavior:
+
+- Input must be a valid existing seriatim output artifact (`seriatim-minimal`, `seriatim-intermediate`, or `seriatim-full`).
+- Raw WhisperX-style JSON is rejected.
+- `render` does not execute merge/trim/normalize transformations.
+- Markdown output is deterministic for the same input artifact and render flags.
+- Category names are not printed directly; `background`, `backchannel`, and `filler` only influence italics.
+
 ## Common workflows
 
 Merge with a speaker map and report output:
@@ -160,6 +189,15 @@ go run ./cmd/seriatim normalize \
   --output-file /tmp/seriatim-example-normalize-object.json
 ```
 
+Render an existing artifact as Markdown:
+
+```sh
+go run ./cmd/seriatim render \
+  --input-file examples/render/input-intermediate.json \
+  --output-file /tmp/seriatim-example-render.md \
+  --format markdown
+```
+
 ## Exit and errors
 
 - Commands return exit code `0` on success.