fix: handle BEADS_DB pointing to directory instead of file

[cwalv]

Summary

• Fix FindDatabasePath() to detect when BEADS_DB points to a directory and delegate to findDatabaseInBeadsDir(), matching BEADS_DIR behavior
• Add regression tests for the directory and trailing-slash cases

Fixes #2548

Problem

When BEADS_DB is set to a directory path (like .beads/), FindDatabasePath() returns the directory itself. The caller at main.go:476 does beadsDir := filepath.Dir(dbPath), which strips one level too many — yielding the parent of .beads/ instead of .beads/. This causes:

• ResolveDoltDir() to create a stray dolt/ directory at the parent level
• A second dolt server to start in the wrong location
• "database not found" errors on all subsequent commands

The BEADS_DIR code path works correctly because findDatabaseInBeadsDir() returns .beads/dolt (a path inside .beads/), so filepath.Dir() yields .beads/.

Fix

When BEADS_DB points to a directory (not a file), treat it like BEADS_DIR by calling findDatabaseInBeadsDir() to look for the database inside it. This is a 6-line change in internal/beads/beads.go.

Test plan

• TestFindDatabasePath_BEADS_DB_Directory — verifies filepath.Dir(FindDatabasePath()) resolves to .beads/ for both BEADS_DIR and BEADS_DB-as-directory (failed before fix, passes after)
• TestFindDatabasePath_BEADS_DB_DirectoryTrailingSlash — verifies trailing slash doesn't bypass the fix
• All existing FindDatabasePath tests pass (-short mode)
• golangci-lint passes with 0 issues