feat: add project rename support

[scion-gteam]

Summary

• Extends the existing PATCH /api/v1/projects/{projectId} endpoint to accept a slug field, allowing projects to be renamed
• Adds scion project rename <project> <new-name> CLI command that updates both display name and slug
• When the slug changes, associated group slugs (project:<slug>:agents, project:<slug>:members) and hub-managed filesystem paths are migrated automatically

What changes on rename

Resource	Impact
Project slug (DB)	Updated to new slug (sanitized via api.Slugify)
Project name (DB)	Updated to new display name
Group slugs	project:<old>:agents → project:<new>:agents (same for :members)
Hub-managed workspace dirs	~/.scion/projects/<old> → ~/.scion/projects/<new>

What does NOT change

Resource	Why
Project ID (UUID)	Immutable — agents reference projects by ID, not slug
Agent records	FK references project by ID
Git remote	Independent of slug
Running agents	Retain old slug until restarted; new agents use updated slug

Slug conflict handling

If the target slug is already taken by another project, the endpoint returns 409 Conflict. The slug is sanitized through api.Slugify() before comparison, so "My Project" becomes "my-project".

Closes #79

Test plan

• TestProjectRenameSlug — rename both name and slug, verify DB update
• TestProjectRenameSlugConflict — attempt rename to existing slug returns 409
• TestProjectRenameSlugOnly — rename slug only, name unchanged
• TestProjectRenameSlugSanitized — slug with spaces/uppercase is sanitized
• TestProjectRenameSameSlugNoOp — same slug is a no-op (no migration triggered)
• TestProjectRenameGroupMigration — agents and members group slugs are migrated
• All existing project tests pass (no regressions)