cli subpackages: delete dead code and its dedicated tests

All unreachable from the CLI entry points (x/tools deadcode + reference grep). Highlights:

• auth/repo_token.go removed whole — repo-scoped token minting lives in internal/entireclient/repocreds; the live resolveContextForCluster seam moved to control_plane.go, and stale comments in repo_mirror* now point at repocreds.
• checkpoint/blob_resolver.go removed whole (test-only; prod fetches blobs via the BlobFetcher path).
• checkpoint/remote: the unused CatFiles batch reader and its plumbing; DeriveCheckpointURL wrapper — its derivation coverage moved in-package to a new TestDeriveCheckpointURLFromInfo against the live private function before deleting the strategy-package test.
• checkpoint: addDirectoryToEntriesWithAbsPath (superseded by addDirectoryToChanges — its symlink-security tests were repointed at the live function, which had almost no direct coverage), FetchingTree.Unwrap/Files, LookupSessionLog, JoinPrompts.
• agent: registry.Detect; factoryaidroid and geminicli path-based transcript variants (prod uses the *FromBytes forms).
• agentimport.Get, api.ResolveURL, gitremote.ExtractOwnerFromRemoteURL, investigate.IsInvestigateEnvEntry + StateStore.List, logging.LogDuration, the paths-package copies of Claude path sanitizing (live copies live in agent/claudecode), and two review test-only wrappers.

Tests that pinned real behavior were repointed at the live functions (registry completeness, URL joining, prompt round-trip, symlink security, review token hydration) rather than deleted.