Add squad registry add/list/remove for discovery-only peer squads (no inheritance) #1290

Summary

squad upstream add <path> exists, but there's no companion command to register a peer squad for discovery-only (no inheritance). Users who want to discover + delegate to another squad without inheriting its skills/decisions/wisdom must hand-edit .squad/squad-registry.json. The error message from squad discover itself even tells users to "create a squad-registry.json" by hand:

"No squads discovered. Add upstreams with 'squad upstream add' or create a squad-registry.json."

This is a UX asymmetry — upstream has a real subcommand, registry does not.

Proposal

Add squad registry subcommands matching the shape of squad upstream:

squad registry add <name> <path>       # add entry; refuses on duplicate name
squad registry list                    # show all registered peers (also: --json)
squad registry remove <name>           # remove entry

The <path> is the repository root (not .squad/). Squad locates the manifest via the existing readManifest(repoPath) which joins .squad/manifest.json internally (cross-squad.ts:121-132). Validates the path resolves to a readable manifest before writing the entry.

Secondary fix in same PR — clarify path semantics

The docs and the cross-squad SKILL.md both show ../friend/.squad as the path. But readManifest() then appends .squad/manifest.json again, looking up ../friend/.squad/.squad/manifest.json which doesn't exist — discover silently returns nothing. Empirically reproduced just now.

Two fixes:

1. Make readManifest accept BOTH forms: if path ends in .squad, strip it before joining; if not, append as today.
2. Update the SKILL.md and any docs that show the wrong form.

Why not just use upstream?

Upstream is for INHERITANCE — it loads the other squad's skills/decisions/wisdom/routing into the coordinator at session start. For cross-org or cross-team peer relationships where you DON'T want to inherit, registry is the right tool. The mechanisms are intentionally separate (cross-squad.ts:235-250 dedupes by name with upstream priority).

Acceptance

• squad registry add/list/remove shipped
• Path semantics accept both repo-root and repo-root/.squad
• cross-squad SKILL.md updated to show correct path form
• squad discover "no squads discovered" hint updated to mention squad registry add
• Tests covering the three new subcommands + dual-path semantics
• Changeset