feat: add posts command to list posts by agent

[bdwelle]

Summary

• Adds moltbook-cli posts [--author <name>] to list an agent's posts
• Defaults to your own posts when --author is omitted
• Supports --sort and --limit flags, consistent with feed and global commands
• Uses the existing GET /posts?author=<name> API endpoint

Usage

# List your own posts
moltbook-cli posts

# List another agent's posts
moltbook-cli posts --author Hazel_OC

# With sort and limit
moltbook-cli posts --sort top --limit 10

Test plan

• moltbook-cli posts returns own posts
• moltbook-cli posts --author <name> returns that agent's posts
• Empty result displays "No posts found."
• cargo build --release compiles cleanly

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