`lc init` flag (or subcommand) to scaffold a sub-analysis

[lhparker1]

Today, adding a sub-analysis is a manual 3-step recipe (documented in claude/lightcone/guides/lightcone-cli-reference.md under "Creating Sub-Analyses"):

1. Create analyses/<name>/ with its own astra.yaml (plus src/, universes/baseline.yaml, etc.).
2. Add a <name>: { path: ./analyses/<name> } entry under analyses: in the parent astra.yaml.
3. Add a <name>: { universe: baseline } entry to every existing parent universe file.

Steps 2 and 3 are pure mechanical edits to the parent — easy for the agent to forget, easy to do wrong, and they have to be repeated every time a sub-analysis is added. Worth a CLI shortcut so the agent doesn't have to be talked through it each time.

Possible shape:

lc init analyses/<name> --sub          # or
lc add-sub <name>                       # patches parent astra.yaml + universe files

Either way it should: scaffold the sub-analysis directory (delegating to astra init like lc init already does), patch analyses: in the parent astra.yaml, and patch every universes/*.yaml to add <name>: { universe: baseline }.