feat(init): treat bare slug as new project name when not found

Previously, `sentry init hello` would throw a ResolutionError when no
project named 'hello' existed. Now it treats the bare slug as the name
for a new project to create:

- Bare slug found (1 match): use existing project (unchanged)
- Bare slug not found: pass as new project name to wizard; org resolved
  via standard cascade (DSN/env/config/single-org/interactive prompt)
- Bare slug matches org name: treat as org-only (like `sentry init org/`)
- Bare slug in multiple orgs: disambiguation error (unchanged)

When the wizard's createSentryProject resolves the org and discovers
the project already exists, it prompts: 'Use existing or create new?'
(--yes mode: use existing silently).

Also documents the 4-mode target resolution convention in AGENTS.md
(auto-detect, explicit, org-all, project-search) including the
project-search-first rule and init's create-on-miss special case.

Author: BYK