Add a page `list`/`search` command for page discovery

[willkg]

Add a command to discover existing Confluence pages — by space, by parent, or by title/CQL — so a user or an agent can answer "what pages already exist here?" before creating (and avoid duplicates).

Why

Today there's no user-facing way to enumerate or find pages; SearchPagesByTitle exists only internally (used by create's duplicate check and fix's title lookup). An agent in Claude Code planning a publish needs to see the current tree/space first — e.g. to pick a parent, detect an existing page to update instead of create, or reconcile.

Shape (to refine)

• list under a space and/or parent (children of a page), and/or search by title or CQL.
• Emit id / title / space / parent / url per hit; first-class --json (array of results in the standard envelope).
• Reuse the client's _links.next pagination that SyncAttachments/listing already rely on.

Open questions

• One command with flags (--space, --parent, --query) or two (list vs search)?
• Expose raw CQL, or a curated subset (title/space/parent) to keep it safe and simple?
• Depth for --parent (direct children vs full descendants; note getConfluencePageDescendants semantics).

Notes

• Complements info (single page) and the export subcommand (Implement an export subcommand (page + attachments to the filesystem) #37).
• Improves Claude Code use: discovery is the missing read-side primitive for planning a publish.