Add legacy sdpub migration (#85)

Author: Moskize91

data/help/commands/legacy.jinja

@@ -0,0 +1,18 @@
+Help Type: command
+Command: wikigraph legacy
+
+Purpose:
+  Low-frequency migration tools for old released archive formats.
+
+Usage:
+  wikigraph legacy migrate <path.sdpub> [--output <path.wikg>]
+
+Commands:
+  migrate    Convert a released legacy archive into the current `.wikg` format.
+
+Examples:
+  wikigraph legacy migrate ./book.sdpub
+  wikigraph legacy migrate ./book.sdpub --output ./book.wikg
+
+See Also:
+  wikigraph legacy migrate --help

data/help/commands/legacy/migrate.jinja

@@ -0,0 +1,19 @@
+Help Type: command
+Command: wikigraph legacy migrate
+
+Purpose:
+  Convert a released legacy `.sdpub` archive into the current `.wikg` format.
+
+Usage:
+  wikigraph legacy migrate <path.sdpub> [--output <path.wikg>]
+
+Input:
+  Supports `.sdpub` archives produced by released SpineDigest v0.1.8 or v0.2.2.
+
+Output:
+  If `--output` is omitted, the output path is the input path with `.sdpub` replaced by `.wikg`.
+  The input archive is not modified.
+
+Examples:
+  wikigraph legacy migrate ./book.sdpub
+  wikigraph legacy migrate ./book.sdpub --output ./book.wikg

data/help/commands/root.jinja

@@ -69,6 +69,7 @@ Help contract:
   - Append `--help` to commands and subcommands to see command-specific usage.
   - Use `wikigraph help <topic>` for cross-command concepts, constraints, and runtime behavior.
   - Treat `wikigraph --help` as the root page of the same help system.
+  - Old archive migration: `wikigraph legacy --help`.
 
 Help topics:
 {% for topic in helpTopics %}